I wonder if it's because of export. If one puts an env var into a command alone then it should be OK. For example, `MY_VAR=1 my-commmand.sh` should leave env vars unchanged for everything else.
Yes that should work. In our case we have a few different windows/panes that all want the same environment so it made sense to reach for export as a way to ensure the environment was set but then ran into this quirk of tmux