TYPO3 Core

I am still unable to reproduce the problem.
The UC filed in DB is of type MEDIUMBLOB, which means 16MB size.
The massive increase in your case is a bit weird, because in my instances with a small amount of BE user groups and a system with 200 groups, the size not change.
In my case the UC filed contains ~15.000 bytes.

I guess it is something else, maybe an extension which produce the problem.
Is it possible that you check the content of the first, second and third change and create a diff?

After some debugging i see reflection informations and warnings in the uc like the following:

Returns the specified configuration.
 The actual configuration will be merged from different sources in a defined order.

 You can get the following types of configuration invoking:
 CONFIGURATION_TYPE_SETTINGS: Extbase settings
 CONFIGURATION_TYPE_FRAMEWORK: the current module/plugin settings
 CONFIGURATION_TYPE_FULL_TYPOSCRIPT: a raw TS array

 Note that this is a low level method and only makes sense to be used by Extbase internally.

or

subGroups";O:55:"TYPO3\CMS\Extbase\Persistence\Generic\LazyObjectStorage":13:{s:64:" TYPO3\CMS\Extbase\Persistence\Generic\LazyObjectStorage warning";s:240:"You should never see this warning. If you do, you probably used PHP array functions like current() on the TYPO3\CMS\Extbase\Persistence\Generic\LazyObjectStorage. To retrieve the first result, you can use the rewind() and current() methods.";s:13:" * dataMapper";

s:46:" TYPO3\CMS\Extbase\Reflection\ClassSchema tags";a:1:{s:8:"internal";a:1:{i:0;s:59:"only to be used within Extbase, not part of TYPO3 Core API.";}}s:58:" TYPO3\CMS\Extbase\Reflection\ClassSchema injectProperties";a:0:{}s:55:" TYPO3\CMS\Extbase\Reflection\ClassSchema injectMethods" 

{i:0;s:35:"http://forge.typo3.org/issues/36820";}s:6:"return";a:1:{i:0;s:46:"array Names of the properties to be serialized";}s:8:"internal";a:1:{i:0;s:59:"only to be used within Extbase, not part of TYPO3 Core API.";}}s:11:"annotations";a:0:{}s:8:"isAction";b:0;s:11:"description";s:388:"Serialization (sleep) helper.

 Removes properties of this object from serialization.
 This action is necessary, since there might be closures used
 in the accordant content objects (e.g. in FLUIDTEMPLATE) which
 cannot be serialized. It's fine to reset $this->contentObjects
 since elements will be recreated and are just a local cache,
 but not required for runtime logic and behaviour.";

s:17:"includeGroupArray";a:0:{}s:14:" * TSdataArray";a:4:{i:0;s:1297:"options.enableBookmarks=1
            options.file_list.enableDisplayBigControlPanel=selectable
            options.file_list.enableDisplayThumbnails=selectable
            options.file_list.enableClipBoard=selectable
            options.file_list.thumbnail {
                width = 64
                height = 64
            }
            options.pageTree {
                doktypesToShowInNewPageDragArea = 1,6,4,7,3,254,255,199
            }

            options.contextMenu {
                table {
                    pages {
                        disableItems =
                        tree.disableItems =
                    }
                    sys_file {
                        disableItems =
                        tree.disableItems =
                    }
                    sys_filemounts {
                        disableItems =
                        tree.disableItems =
                    }
                }
            }

[GLOBAL]

    options.saveDocView = 1
    options.saveDocNew = 1
    options.saveDocNew.pages = 0
    options.saveDocNew.sys_file = 0
    options.saveDocNew.sys_file_metadata = 0
    options.disableDelete.sys_file = 1

[GLOBAL]

    options.saveDocNew.tx_gridelements_backend_layout=1

[GLOBAL]
options.pageTree.doktypesToShowInNewPageDragArea := addToList(142)";i:1;s:23:"admPanel.enable.all = 1";i:2;s:137:" 
                            // Setting defaults for sys_note author / email...
                        TCAdefaults.sys_note.author =
                        TCAdefaults.sys_note.email =
                    ";i:3;N;}s:14:" * userTS_text";s:1487:"options.enableBookmarks=1
            options.file_list.enableDisplayBigControlPanel=selectable
            options.file_list.enableDisplayThumbnails=selectable
            options.file_list.enableClipBoard=selectable
            options.file_list.thumbnail {
                width = 64
                height = 64
            }
            options.pageTree {
                doktypesToShowInNewPageDragArea = 1,6,4,7,3,254,255,199
            }

            options.contextMenu {
                table {
                    pages {
                        disableItems =
                        tree.disableItems =
                    }
                    sys_file {
                        disableItems =
                        tree.disableItems =
                    }
                    sys_filemounts {
                        disableItems =
                        tree.disableItems =
                    }
                }
            }

[GLOBAL]

    options.saveDocView = 1
    options.saveDocNew = 1
    options.saveDocNew.pages = 0
    options.saveDocNew.sys_file = 0
    options.saveDocNew.sys_file_metadata = 0
    options.disableDelete.sys_file = 1

[GLOBAL]

    options.saveDocNew.tx_gridelements_backend_layout=1

[GLOBAL]
options.pageTree.doktypesToShowInNewPageDragArea := addToList(142)
[GLOBAL]
admPanel.enable.all = 1
[GLOBAL]

                            // Setting defaults for sys_note author / email...
                        TCAdefaults.sys_note.author =
                        TCAdefaults.sys_note.email =

[GLOBAL]
";

The hole serialized array contains a lot of those messages.

If I remove the subgroup relation between the groups, then the uc-field does not inflate.
Here are the relations shown:

I can't see a problem within it (e.g. no cycles or something else).

we're reproducing the bug here, with the same TYPO3 Version, we can filter once after reseting data, then, "An exception occured while executing "UPDATE" 'be_users'' SET ''uc'' = ? WHERE 'uid'' = ?''

• TYPO3 v9.5.7
• PHP 7.3.2
• MySQL 5.7.22

On 2-3rd time of filtering I get "An exception occured while executing "UPDATE" 'be_users' SET 'uc' = ? WHERE 'uid' = ?....."

@Florian Kuss Schuhmann thank you for your feedback;

I close this issue as solved with #86361; If you think that this is the wrong decision or experience the issue again, please reopen it or ping me.

Thank you and best regards.