Hi @Ivan_Groenewold - Happy New Year and thanks for your offer. I managed to fix the problem by a massive restore of all databases using a full mysqldump with --ignore-table option to exclude the broken table. The whole thing caused a 1.5h downtime for my customers, but it was worth it, as anyway I wanted to get ready for XtraBackup with a clean state of all InnoDB tables.
I have upgraded to Percona Server for MySQL 8.0.22 on Dec 14th, which supposedly fixes MySQL Bugs: #99791: MySQL 8 orphaned table due to unchecked non-existent row format check.
This issue did not pop up on any other table.
Thanks for your support. I guess you can close this thread.