Does a mobile/runtime geodatabase (.geodatabase) not support all the same SQL functions as a File GDB?
I have the same data with the same label classes in a File GDB and then published to AGOL as a hosted feature service. The CHAR_LENGTH SQL function evaluates fine in each of these, but as soon as I create an offline replica (local copy/download map) from Pro, it breaks.
Hosted Feature Service:

Offline Replica:

Thanks,
Walker