When accidentally creating a loop in the semantic model for a Group Item (made it member of itself while mass-editing) with an .items file, no error/warning was logged. Only when I selected the Locations-tab in MainUI an error was displayed:
A a loop has been detected in the semantic model: alarmSystem is both descendant and parent of alarmSystem. Please correct and refresh.
BTW, also notice the grammar error in the message (“A a loop”). 
For completeness, the offending line in the .items file was:
Group alarmSystem "Alarmsysteem" <siren> (alarmSystem) ["AlarmSystem"]
I would have suspected the check to be done at file load time. Created an issue.
UPDATE: It looks like when removing the offending group membership from the Items file and triggering a reload by saving it, an error entry appears in openhab.log:
ERROR] [s.internal.SemanticsMetadataProvider] - Recursive group membership found: alarmSystem is both, a direct or indirect parent and a child of alarmSystem.
@Lolodomo I noticed you worked on the logic to only remove/re-create an item defined in an `.items file if it has changed. Could it be related to that change and only validated under specific conditions?