Hi,
Many thanks for the feedback! We've tried to reproduce the bug the way you mentioned (it is very easy to follow), but unfortunately to no avail yet. This means there's a weirder underlying issue that manifests the bug when the game is being run under specific conditions (e.g. Linux). That said, we will further investigate the issue and try to come up with some solution (for one, even if the + button remains active, we can ensure nothing happens when you click it when it's 5/5).
Thank you again, happy hunting!