Okay. I'm sorry you're stuck with this, but unfortunately I can't provide a quick fix. :( It seems like it might have something to do with setup, rather than a bug in the code (in other words - I can't reproduce this on my machine). But I'll look into it and ask around on the Renpy discord.
Can you tell me if the red boxes included in the image were drawn by you or shown in-game? Did the same issue crop up elsewhere? Does it only show up if the game shows a choice menu (like the one in the screenshot) or does it affect all buttons?