B7sus/C would add an A that isn't in the Cmaj7b5 chord.

Bsus/C, no 7, would spell out from bottom to top as C,B,E,F# and would at least contain the same notes. Likely would sound best if the Style used an Open Voicing...

This might be a case where BiaB is being given a chord that doesn't make sense to the underlying ruleset. Often is the case where the *location* of a given chord within the other chords around it seems to have something to do with how BB handles these things.

--Mac