Looks like this is upstream issue #2438: https://github.com/tmux/tmux/issues/2438
It was fixed on master in the 3.1b timeframe, but unfortunately the backport was missed and the fix is not included in 3.1c. I can probably get the fix included in a stable update. Alternatively, tmux 3.2a will be available via bullseye-backports in the near future.
