I use the openstreetmap vector map layer when making courses at connect.garmin.com. I've noted that some of our big lakes in Sweden disappear when you zoom in, like lake Storuman, Torneträsk, Uddjaure and Storavan, and also parts of Hornavan. I haven't looked elsewhere in the world but I'm assuming it's a problem with many big lakes all over.
Other openstreetmap map providers with vector tiles like maptiler doesn't have this problem so I don't think it's a OSM data problem, but a bug in the map Garmin uses.