Using a custom powerline theme breaks command line when pasting text and moving the cursor

[moshpirit]

I have a custom theme based on Powerline (link to the theme files on my nextcloud account). When I paste anything on the terminal, it breaks the command. In this case, let's say I want to play a youtube link with mpv, if I type mpv <url> and move the cursor back (with any terminal), I get this:

It seems like the characters 'ht are duplicated, but it actually displaced the three first characters to the right; in reality there's something more like this:

What am I missing and how can I fix it?