I'm using powerline-go to generate the bash prompt. It works fine in the tilda (also Konsole), like so:
Basically, the segment seperators end up shown as underscores. This is pretty annoying, and I'm not able to understand why tmux refuses to render my fonts the same way as a regular terminal.
I'm using Fantasque Sans Mono, but I see the same behaviour with other fonts.
In fact, it seems tmux doesn't render any utf-8 symbols properly. I tried
echo -e '\xe2\x82\xac'
this prints the euro sign in the normal terminal, but again an underscore in tmux.