I played with this before (e.g. here) and in short, RouterOS does not like nulls in L7.
If you try to put null in regexp using \x00, it does not allow that. It does allow to enter \0, but I’m wondering what it might understand it to be. It definitely does not match nulls here.
Also when trying to match using L7, it first drops all nulls and tests the result. So to find any A record question for any domain, this should work ok:
[a-z]\x01\x01$