The callback function assigned to tag ASLFR_HookFunc is not invoked when tag ASLFR_Flags1 contains flag FRF_INTUIFUNC.
Therefor it currently seems not possible to use one single hook function to handle both filename filtering (FRF_FILTERFUNC) and IDCMP window messages (FRF_INTUIFUNC).
It is documented to work as described above (even though RKRM documentation is not in line with newer ASL library) and works as expected on 3.x.
Although not tested myself, it would not come as a surprise if a ASL font requester also does not invoke the callback function for (probably) the same reason(s).
Fixed by commit #54927.