I reported thisa few months back. https://forums.garmin.com/sports-fitness/running-multisport/f/forerunner-965/326215/support-for-custom-maps
It seems that nothing has changed. I just installed 2 OSM maps, one is shown in map manager on watch, the other not. Both worked fine on my Fenix 6 Pro. What is wrong with Garmin that such basic functionalities are completely broken in 955?