Hi @Simon-q
The issue with waypoint names is as @Marinus van Deudekom says, it is a Garmin thing
the XT specifically renames points in a seemingly completely random way, it even can change the location by small amounts each time you rename the point !
I don't believe it is to do with the position of the points but more to do with the Garmin/Here database for the road names not matching or somehow conflicting with your name of the point.
The Garmin tries helpfully to "correct" what it thinks is an error but is in fact deleting/corrupting hours of your work
The only way round it I know of is to check the points after they are loaded to the Garmin & rename the ones that have changed back to the name you original chose.
Transfering via Basecamp or such does not help, the names are changed as they are loaded onto the device