tmux is amazing and you should really consider adding to your workflow if you work from the terminal. However, every once and a while it can mess with things in a unexpected ways.
For me, I was messing with the impressive Ionic Framework and ran into a quick and lucky easy to solve issue. Apparently launching the iOS simulator from a tmux session can be problematic. The simulator was launching for me but then never doing any. A black screen of death without any notices or errors.
Turns out you just need to launch from a normal terminal window. See the link below. Not a huge deal really. For my workflow, I just switched from iTerm2 to terminal for just launching emulation. Actually kind of like having the 2 apps considering their purposes are pretty different.