It seems for no reason the indicators disappeared from the panel.
More precisely, a grey blank space is there which behaves like an object on the panel (it can be left clicked, and moved, deleted, locked like other panel elements). But it is empty. This happens with the full indicator applet too.
I tried reinstalling the mate-applets and mate-applets-common packages but it didn’t solve it.
EUREKA!
Found out what it was. In Mate Tweak I had somehow turned off “Enable Indicators” in the Interface section.
The clues were from a post and picture that had a “Panel” section in Mate Tweak - my install doesn’t, the panel options are inside the Interface.
(I had asked this also, not seeing your post till after.)
I could solve it too with selecting a panel layout. Adding them is now possible and the tick box works again!