It's possible that the back button is just off-screen depending on what aspect ratio you are playing on. I think this was fixed in the new version.
Viewing post in Monster Souls: Chains of Chimera comments
That does indeed seem to be the case. I've done further testing and not encountered it on most other aspect ratios, albeit I have enough experience in coding to know niche cases are to be expected. As a precautionary measure, it might be good to add a keybind to forcibly close the menu/shop during playtesting, but overall you've done a very good job avoiding game-breaking bugs.