Can't record a physical audio input and bus simultaneously

This has been driving me nuts, any help would be most appreciated. Here's the setup:

Track A routes to Bus 1, track B has Bus 1 as an input, and track C has an arbitrary in on my interface as it's input. I arm B and C, both show levels being input. While recording, both B and C are drawing the recorded waveforms, but as soon as I hit stop, I'm left with a completely empty region on track B, and a normal one on track C. If I disarm C and record, B records perfectly. Track A can be whatever, audio, software instrument. I've recreated this on both my MOTU 828mk3 and my Macbook's built in soundcard. Anybody else experience similar issues?