this can be the result of one of two restriction of the Garmin IMG file format on Garmin GPS units.
different Maps need a unique combination of FamilyID and ProductID of the map. i.e. at least one of the ID’s must be different for each map stored on the Garmin Handheld (The same is true for basecamp so this is probably not the case here).
All file names of the map tiles i.e. the numbers given to all single tiles and the “overview”-map must also be unique globally, i.e. no tile number can be reused in another map also stored on a Garmin GPS unit.
That means if a map you uses the number 62400001.img to 6240010.img for its tiles and overview map these numbers MUST not be used in any other map you intend to use/store at the same time on the Garmin GPS unit.
basecamp seems not care about this restriction. So the maps appear to be correct if you view them in basecamp/MapSource but are not displayed or rather only one of them is displayed if both are on the GPS unit.