Mixer board gain sliders control extra tracks, behave badly

Just upgraded to Audacity 2.3.1, excited to go 64 bit. On MacOS Mojave 10.14.3. But the mixer board has gone insane.

This problem occurs if I start a new project and import two mono WAV tracks.

The gain sliders on the mixer board seem to control the proper track but also one to its right. (Though I don’t see the slider’s effect on the track in the main window until I hover over that track.)

Controlling the mix from the main window seems to work, but the track level meters on the mixer board work strangely. For example, right now, according to the main window, I’m soloing TRACK01 panned hard left. It sounds like that. And the level meter on the main window agrees. But on the mixer board, both left and right level meters on that track are bouncing as if there were audio on both left and right. But that behavior is inconstent.
In general, the rightmost track on the mixer board seems to work correctly.

This has never happened before 2.3.1. Is there some new feature or default setting I haven’t noticed, or is this a bug?

Thanks for the report. I can confirm it’s a bug.
I’ll log it on the bug tracker.