Yeah, thanks for the tests and quick feedback. The diagnosis seems to indicate with a good degree of certainty what I feared: two plugins are modifying the same menu class at the same time, and one is overwriting the other. The change in loading priority order indicates this.
It's as if my plugin tells the game to look left and VisuStella to look right; the latter order is what prevails.
The fact that VisuStella still has two other plugins that are dependencies only complicates things, because I'm not sure if only the final plugin is modifying the class or all of them.
Can I talk to you on Discord or via email? I'll need a code snippet to try modifying a version of my plugin to add the menu instead of creating it. This will be a version modified especially for you because, as a rule, I can't guarantee compatibility with other plugins that modify the exact same class in the same way as my plugin (it would be a development nightmare to guarantee that).
But we'll try to make it work.