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We do the opposite here - before upgrading the Server, all users must upgrade their machines. Today, I personally am on 2018.4 but a few others aren't, so our Server is on 2018.3. When I went to publish an app to the Gallery from my machine (so a 2018.4 app to a 2018.3 Gallery), I got an error stating the Gallery has to be upgraded (meaning the Server).
Since both Gallery and Server are in that Mongo instance, I'm guessing that's what you're running into as well.