TYPO3 Core

Patch set 1 for branch 10.4 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/68318

Alright, I see. The issue is not that the validation per-se is broken, but there is (for whatever reason) a limit of maximum 20 allowed items defined in TCA. A quickfix to solve this in your case is to override that specific TCA:

unset($GLOBALS['TCA']['be_groups']['pagetypes_select']['config']['maxitems']);

Andreas Fernandez wrote in #note-7:

Alright, I see. The issue is not that the validation per-se is broken, but there is (for whatever reason) a limit of maximum 20 allowed items defined in TCA. A quickfix to solve this in your case is to override that specific TCA:

Ah. We have like 35 custom page types. Thanks, I'll include that in case it ever breaks again.