I can add the missing tile, but then the map installs as “OSM generic routable” and replaces the existing “OSM generic routable”. Since for some of my work I use a map of Hispaniola that is the Haiti (or Dominican Republic) map plus the rest of the tiles needed, also installed as “OSM generic routable,” this is a problem.
One solution would be to install a downloaded map with a name I choose, but I can’t see how to do this.
Where did you download the garmin map for Jamaica from? There are lots of people offering Garmin downloads (see e.g. https://wiki.openstreetmap.org/wiki/OSM_Map_On_Garmin/Download ). You’ll need to raise the issue with whoever you downloaded it from - and of course you could try a different source to see if that has the same problem.