TYPO3 Core

IMO setting the tablenames field is not needed here at all, because a few lines down the MM_match_fields are applied, which includes the tablenames field if applicable.
The use case that is mentioned in the comment (tt_content.category) produces the following query (notice the duplicate tablenames):

DELETE FROM `sys_category_record_mm`
WHERE (`uid_foreign` = 42)
    AND(`tablenames` = 'tt_content')
    AND(`tablenames` = 'tt_content')
    AND(`fieldname` = 'categories');

This is what it behaves like in TYPO3 10.4.11 at least - but from looking at the code it will be the same in master.

Damn. I saw this on Tuesday already when I re-reviewed some of the v10 release patches again. Wanted to fix it already and did not expect anyone would stumble upon in so quickly ...

The main issue is that db structure of MM tables is not fully documented and nailed and some TCA details are missing or are implicit.

Existence of 'tablenames' field for instance is implicit only through the MM_match_fields definition. But it is also hardcoded in RelationHandler without TCA check at at least one place - this is why i used it this way in the patch.

I'll come up with a bugfix to relax this part a bit again (I hope until Tuesday).

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/c/Packages/TYPO3.CMS/+/67188

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/+/67195