The issue only occurs for records created/loaded via AJAX; if a record is already stored and thus opened on editing the parent record, the tree is rendered just fine.

I just tried and it fixes loading of trees within inline records. But at the same time checking items doesn't work at all. Already checked items are kept but you cannot check items. If you uncheck a checked item, you cannot re-check it again.

Attached Patch is working for me.
The second issue mentioned by Mathias seems to be resolved with #75947 in the meanwhile.

Tested with 6.2.25 - can we bring it to next patchlevel?

seems that the test extension is broken.
I'm getting "'Unknown column 'related_category' in 'field list'' "

 lastBuiltQuery => 'SELECT uid,sys_language_uid,l10n_parent,parent,items,images,single_pid,short
      cut,related_category,deleted FROM sys_category WHERE uid=257 LIMIT 1'

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/49684

Patch set 3 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/49684

Testing instructions¶

- install the attached extension
- edit category record
- add inline relation at the bottom of the record (related_category field)
- without the patch, the category tree is not rendered in the inline child

Patch set 4 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/49684