Here is a curious behavior:
When I click on the formatted text, it reverts to standard formatting:
When I click anywhere else, it goes to a serif italic again.
On MacOS 10.14.6.
I thought maybe it was a rich text issue, but it's not— as I explain below.
The problem occurs between dollar signs.
I hit return.
It's a known problem, and fortunately already fixed in the current 1.6 beta! You can either wait for 1.6 to be released or get the beta versions directly from the GitHub page: https://github.com/Zettlr/Zettlr/releases