Check the rigidity of the engine mount. Mounts that aren't rigid enough allow the engine to rotate (vibrate) along the crank axis while running and the throttle barrel bounces around in the bore. This wears or breaks the retaining screw. The barrel is usually full hard so it probably won't wear.

Pe, is 1mm of end gap is acceptable in a <25mm bore engine? An OS FS-91 begins life at about .003", 0.08mm gap. A Magnum FS-91 I measured after 400-500 hours had a gap of .012", 0.3mm, the OEM replacement ring gap was the same! The bore was fine. I happen to have a NIB Magnum 91 at the time and it also measured 0.3mm.