Hello @BrittanyBurson, hello @BruceHarold , thanks for your responses!
The feature service target field is defined as a string(20) in both: FME writer and hosted feature service.
The layer was initialised by uploading a FileGeodatabase into AGOL. After upload the data is daily updated by FME workflow.
It exists as a hosted feature layer since 26th Jan 2021. The problem occurred at the beginning of this week for the first time. Nothing changed during our FME workflow since 3rd December.
Unfortunately I can not delete the existing layer because of Covid-19 related contents. Maybe I'll create a second feature layer with the exact same data and datatypes, testing my updating workbench/workflow and replace the the datasources in the linked apps and maps via AGOL assistant.
It's frustrating that something in FME or AGOL changes string data. A taling zero looks kind of weird for active Covid cases ...