TYPO3 Core

Can confirm this for TYPO3 7.6.2. The group relation gets lost, even when exporting the group, that is owner of the page, alongside the page itself.

see typo3_src/typo3/sysext/impexp/Classes/ImportExport.php:2184

// Have to reset the user/group IDs so pages are owned by importing user. Otherwise strange things may happen for non-admins!
unset($this->import_data[$table][$ID]['perms_userid']);
unset($this->import_data[$table][$ID]['perms_groupid']);

I am not really sure if this should be changed

Actually strange things happen, because the user/group is unset. Additionally, you aren't notified about that fact so it needs investigation that the IDs have to be re-set after import.
It was OK to leave the group IDs intact for the last ~200 releases of TYPO3, so why no more? Have there been complaints the other way round?