Looks like the latest Mac version of BaseCamp (Version 4.8.13) doesn't update the USERDATA file on the GPSMap's SD card when waypoints are added. I noticed there's no change in the time stamp of the file when the waypoints are supposedly transferred to the file. The GPSMap doesn't see the additional waypoints, and when the SD card is re-read by BaseCamp, the added waypoints aren't there.
However, it will update the USERDATA file if a waypoint is deleted. And I did notice that the time stamp of the file changed when that was done.
Tried it with two different Macs with the same results.
When I did this with BaseCamp on a Windows system, the USERDATA file was properly updated with the new waypoints. So BaseCamp on Windows works as it should.
Also worth noting is that it works when I tried it with a Garmin Montana 600 handheld GPS. Waypoints get added when they're transferred to the device.
So it's not the GPSMap device. It's not a particular Mac. And it's not the waypoint data. It's definitely BaseCamp not updating the USERDATA file.
I'm running Mac OS 15.2 on an Apple M3 as well as an Apple M2 Ultra.