Android version of app reverts to older version

One of our customers is complaining that the Android version of the app is reverting to an older version after some time. They describe it as “after a few days” but it could just be after the app is closed.

I don’t have any deploy plugin or “cordova-plugin-ionic” which according to other posts, seems to have been the culprit for many people.

Does anyone have any idea what might be causing this? I can’t reproduce this on my device but they are adamant that they’re doing nothing wrong. They’ve uninstalled the previous version, then install the new version, and after a few days, they get the old version back.