@anticapitalista: Hum... I do have two work-arounds, but they are both ugly;
1- add a hook so every time the used does a "sudo apt upgrade", also performs a "install --reinstall desktop-session-antix". I'm assuming this happens during some upgrades, I may be wrong (my main computer's menu does not show, by choice, the change desktop menu, so I didn't even realize this kept happening until now)
2- add a "Update Desktop list" entry to the Desktops menu that performs a gksudo apt install --reinstall desktop-session-antix, so users can "self-fix" the problem by themselves- if you want to be really obvious, name that entry "Fix the Desktop list)
It's nothing major, but an little annoying bug... way above my "pay grade".
A bit off topic, but related to this:
Users can always log off and use F1 to change desktops... By the way, I remember, long ago, suggesting adding a text embeded into the log in screen default backgrounds, in the top, saying something like " F1 - select desktop", that could be handy when this happens, because, well users faced with a bug, no matter how small, should not have to be told to RTFM, for a way to change the desktop :-)