Just completed an upgrade from 16.04 to 18.04. This is the login screen I get. Looks a lot like the old one but shoved over to the left, where the new one would be. Upon logging in I got an error message saying that Ubuntu 16.04 had encountered a problem but the System Monitor listed the system as being 18.04.
The desktop did not change layout nor did it it add indicator applets.
Rebooting did not fix either issue. Then I opened Mate-Tweak and changed the layout from Familiar (which wasn't in fact displaying) to Traditional, that made the indicators show up again.
Is there a log or something I can get to you? If so let me know what and where and I will get that to you before I install 17.10 and see how an upgrade goes.
EDIT: Also, encountered this issue while upgrading.