When I add a note to a Via Point then export the route to either gpx (then to MapOut) or to Garmin the note appears as expected on the Via Point but also on the route end. If more than one Via Points have notes it looks like the last Via Point note is also used on the route end point. Happening on every route with notes on via points eg

https://cycle.travel/map/journey/1045589

That it is happening both when gpx passed to MapOut as well as when route sent to Garmin makes me think might be in cycle.travel