Yes I found this bug too. The bug arose after I did a clean full install of Mate 18.04LTS, but I re-used my old Mate 16.04 home partition. So it's probably confused by my old style configuration files which were imported from Mate16.04 to Mate 18.04.
The workaround of using Mate Tweak to switch the panel style to something else and then back to Traditional fixed the problem for me.
Yes I found this bug too. The bug arose after I did a clean full install of Mate 18.04LTS, but I re-used my old Mate 16.04 home partition. So it's probably confused by my old style configuration files which were imported from Mate16.04 to Mate 18.04.
The workaround of using Mate Tweak to switch the panel style to something else and then back to Traditional fixed the problem for me.