Environment

Windows build number: Microsoft Windows [Version 10.0.19042.388]
Windows Terminal version (if applicable): 1.0.1811.0
tmux 3.0a
Alacritty 0.4.2

Steps to reproduce

Run tmux in Windows terminal WSL2 (Ubuntu), after a fresh boot into Windows.
Create some horizontal split panes in tmux (works)
Create some vertical split panes in tmux
Run some commands with output (in some panes)

Oddly, the symptom disappears if I close and start Windows terminal again and re-attach to tmux.

Expected behavior

Text output is clear and readable and output in their respective panes.

Actual behavior

The text output from different processes seems to bleed across panes.
This occurs with a vanilla/empty tmux config too.

My TERM is set to xterm-256color outside tmux and tmux-256color in tmux.

This behaviour also seems to happen in Alacritty (0.4.2), which in my understanding shares the same backend (ConPTY?) as Windows Terminal.