Regarding "on the channel" - Channel is the input signal path of a physical mixing console and that's why gain, EQ, panning, aux outs, Fx outs, group outs, faders, and such are called the Channel Strip. A Track, which is the common name for a DAW channel, refers not only to the channel path, but the entire recording path that's inclusive in all DAWs. Technically, a BIAB Mixer Channel isn't as full-featured as a DAW track and I said channel without thinking because it's been driven into my brain for many decades. You made the right choice highlighting and selecting the bass track.

< I opened the Piano Roll view and I could hear the bass sound.
Then I clicked the Playable Track button, but no message (because it's already Hi-Q instrument?)
and I could hear the bass sound this time! >

Perfect. Now we know the path and settings are correct.

< So I created the mute region and edited some bass notes.
But when I played it, the bass notes disappeared............. >

Yes. You're correct and BIAB did what is expected.

Your two results above reflect the definition of Playable RealTracks concept. Underlying midi data on a RealTrack mixer track requires the instrument on that track to be a RealTrack. Activating the Playable RealTrack function, the RealTrack instrument will mute and the muted region sound will come from the Hi-Q midi instrument playing the muted region midi data.

If you still have your test project, freeze the track, load a RealTrack instrument on the bass track you've edited and activated the Playable RealTrack feature and when the song is played, you should hear the Playable RealTrack region playing the midi data and the actual RealTrack audio thereafter. Sound is generated differently for audio RealTracks and Midi instruments. In the test song, a midi style and instruments were loaded by default and playing verified the pathway and settings. In your second statement, the midi was overwritten and disappeared because there was no RealTrack assigned to the track at that time to generate new underlying midi data. Freezing the track preserves any custom midi data you insert onto the track.

BIAB 2025:RB 2025, Latest builds: Dell Optiplex 7040 Desktop; Windows-10-64 bit, Intel Core i7-6700 3.4GHz CPU and 16 GB Ram Memory.