Apple - Terminal leaving a blank line on High Sierra when tabs are used

This issue has been corrected in macOS Mojave 10.14.

Definitely not the ideal solution, but try adding

osascript -e 'tell app "Terminal" to set number of rows of window 1 to 100'

to your ~/.bash_profile. This runs an AppleScript every time you open a new tab. (This may not work for you specifically. I don't know enough about how tmux works)