Because it isn’t in their power to “fix” it. They either use live filters or they don’t.
I’m not sure I buy the explanation though, it is disallowing some bizarre call signs that are unrelated to profanity or historical flight number.
Something odd is going on but it still may be what the filter is returning to them so their hands may be tied.
They ought to at least raise a ticket with the live service though. Beyond that they have bigger fish to fry.
It’s annoying but meh.