I just tested the behaviour of RP loose mode and it considers default route as normal route. In case default route is installed, loose mode has no sense.
I tried the same on Arista switch and they correctly ignore the default route.
I can see your point, but the default route is a connected route too (and potentially dynamically changing). But even if Mikrotik definition is "wrong", "fp-filter=loose" has included the default route for a very long time, and changing functionality is risky in V6 – since there may be users with fp-filter=loose that expect the current behavor (even if it's
potentially wrong).
The firewall can do same functionality is likely why there isn't too much interest. I guess fp-filter=loose might be slightly more performant than using the firewall to implement the same rules, but dunno.
I'd imagine V7 be same, but you could try it that and confirm same behavior?