That's not fully accurate. Aux tracks are similar to busses. They can receive signal during fast bounce as well.
Most likely what's happening is that the OP is trying to bounce just the aux tracks without the tracks sourcing it. Since bounce is selection based, if you don't include those in the selection it could be the reason why it is silent.

Well try and improve it in the future by automatically including all source tracks.
For now try including all source tracks.

Noel Borthwick [Cakewalk]
Most likely what's happening is that the OP is trying to bounce just the aux tracks without the tracks sourcing it.

True, but if you select multiple tracks and bounce/export with Source = Tracks, you'll get one track/file for each selected track. The only way to have multiple tracks selected and get one mixed track/file is to go through a bus and set Source = Buses. I believe the OP already found that workaround.