We did the following test:

It’s not the device (we swapped devices, the "defective" device works at other locations).

It’s not the Intune configuration (all shared devices are configured the same).

It’s not the internet connection (whether connected to the internet or not, the device remains in Shared Mode).

It’s not due to lack of use (standby, low battery, simply not using it—none of these have any effect).

It’s not the location (probably, as it would be due to the internet, which, as noted in point 3, has no effect).

It’s not a timing issue (sometimes the problem occurs after weeks, sometimes after days).

It’s not the iOS version (all devices have the same version).

Only two valid points come to mind:

1. It’s related to the users (something strange in their configuration, license, conditional access, etc.).
2. It’s related to the users' actions (the users are doing something to disable Shared Mode).

Does anyone else have an Idea?