[nobleach]

I think this may be an iTerm thing. By default, using the steps you've posted, the highlighted text will not go to a system clipboard.

[jcard]

Ah! Thanks for pointing that out. That's the piece I forgot about here. iTerm is populating the system clipboard from tmux because of the tmux integration [1].

[1] https://iterm2.com/documentation-tmux-integration.html