There seems to be an issue with 2 different sets of data - "map data" and "cache data".
Logged a find today, cache data showed as found, map didn't show a smiley. Went back into cache details, defo showing me in the list of logs, back to map, still showing not found icon.
Went back into cache, swiped down to refresh as BFJ above suggested, back to map view and icon was updated. Not an exhaustive debugging but it worked that time.
Haven't had a waypoint problem again yet but will be interesting to see if a refresh sorts this too.