Per title, I've noticed this issue of late.
Example: an activity recorded on one device, that is automatically added to another devices sync queue as part of the TrueUp feature (ie. activity histories syncing) but is then deleted from the Garmin Connect platform before the second devices sync = obsolete activity now remains in sync queue indefinitely, causing the second device to always show a sync failure.
Same with routes, when using say komoot to plan a route and with the Garmin Connect option to automatically sync these, when you amend a route in komoot it re-sends it to Garmin Connect. If you change the name of the route, it changes the name in Garmin Connect (clever) but within your device sync queue(s) there are now two entries - one for the new course name and one for the old course name (latter which no longer exists, not clever).