Thanks for testing Valentin. I have working on our Vst implementation for another story and from that think I have a pretty good grasp of what the problem is here.
In short, the issue is that Vst 3 plugins have 2 parts, an audioprocessor and a controller, Each having their own copy of the whole set of parameter values. If you set a parameter from within the audio processor, Sushi does not forward the updated value to the controller part of the plugin, which was an oversight on out part.
Can you live with it only working for Vst2 for the moment? We will likely have a fix for the next version of Sushi coming out.
Edit: This was fixed in Sushi version 1.1