XFBR not working when passing Custom Name Function

Yeah, that's not going to work. You can only append ":Name()" at the very end of the filter, e.g. XFBR(...):Name("My ColName") should work. :Name is effectively a command to the cubeview ("rename the column with X"), not a generic function.

In your case, I guess something like this *might* work, although it depends on what your XFBR actually does:

XFBR(XFBR_CubeViews, CustomTimeExpansion, timeExp=T#2020.Base):Name(|MFTimeShortDesc|)