Answer
This kind of add-in version mismatch after an update has occurred before, tied to the hosted add-in package on the Add-ons page still pointing at an outdated build behind the scenes — if you hit exactly this symptom (installs cleanly, but buttons are blank/non-functional in one specific Excel desktop version), check for an updated add-in installer file to manually reinstall (uninstalling the old one first) rather than assuming your Excel installation itself is the problem, and check current release notes for whether this has already been fixed in your update level.