Is there any way to avoid this?
tmux has the
rectangle-toggle command for this. It's not fully explained in the man page; it only appears in the list of commands available during copy mode:
Command vi emacs [...] rectangle-toggle v R
It switches between the line-based selection you demonstrate, and rectangular selection (similar to Vim's blockwise visual mode). In the latter mode, you can start your selection at
static and extend down+right without taking in anything to the left (the line numbers).