FWIW, this was reported upstream[0], and fixed[1], and released as part of 1.5.0a. Either backporting the relevant changes from PR 434 or updating to 1.5.0(a) seems like a good idea.
0: https://github.com/tmux-python/tmuxp/issues/433 1: https://github.com/tmux-python/tmuxp/pull/434 2: https://github.com/tmux-python/tmuxp/releases/tag/v1.5.0a
