Intersting is that if I open the app properties in Android, it shows me at "app-details in Store" that the app was actually side loaded and not connected to the Play Store. The list of installed apps in the Android system settings shows only one Gadgetbridge app - this exact one...
Google Play users have started seeing updates to modified apps.
Many users like to use modifications to popular apps because they bring some additional functionality that improves their experience. And usually, when you install a modified version, the app doesn't appear in the list of available updates.
However, a few days ago, users started seeing mods in the list of available updates. This is because Google activated the server-side flag MyAppsV3__univision_manage_apps_and_device_page, which essentially slightly changes the spacing between items and the size of icons (as seen in the image). But it also has the side effect of displaying available updates for modified apps.
Trying to update a modified app via Google Play will naturally end in an error.
*this only applies to modifications that use the original package name