No explanation at this moment!
But as it works correctly with 5.19 and not 5.21 it looks like we have an issue.
We will research into that now.
Can you procide a 'test case': a complete export with 5.19 and 5.21 respectively and attach. You can create a ticket and attach to the ticket if you don't want to expose the data in public.
Also: does this only and always happen inside 'inner join' constructs?