I've often noticed that when entering "Power chords" that BiaB will play the 3rd.
BTW, I hate the term "power chords".
Two notes (like root and fifth) is by definition an interval, not a chord. It takes 3 notes to play a chord.
Although I hate it, everybody uses it so I'll use it myself, and quietly grimace inside every time I do.

Since I export everything I do in BiaB to a MIDI file, I can erase the 3rds but it is time consuming.
Insights and incites by Notes