Along with persist’s suggestion, if you haven’t tried already, input some audio to the BB’s L & R inputs, either in stereo or mono L or R one at a time, and see if the audio carries through to the outputs.
There was an issue a few years back that I had questioned about stereo wav’s used in the BB and not appearing at the outputs correctly. I believe this was corrected by a firmware fix. I’m not sure if in the input jacks there is a contact that senses when something is plugged in and tells the BB how to configure the outputs if one input is used or both. It may not help, but I’d try a short squirt of DeOxit contact cleaner into the input and output jacks followed by inserting and removing a plug a few times.
Likely different than your problem, but the old input/output thread…