I'm having this problem as well, in version 18.10 (15.0.18390.0), with SQLPrompt installed. Reopening SSMS doesn't help. Occasionally, one level of outlining will appear at the start of a stored proc script. Once the outlining submenu disappeared from the Edit menu.
I've tried altering most of the settings associated with outlining, but nothing has helped.
This is a very useful feature, and I'd really like to be able to use it.