FLStudio.
I also tried IL Minihost Modular (1.5.8 ) and whilst it felt a little better than FLStudio, the problem was still present.
I now suspect that the issue is also present in FLStudio with non-drum VSTs ...
FL internal plugin (say, Sytrus), I can work on the plugin interface, click on another element (the piano roll, for instance) and this brings the other element on the foreground. This doesn't work with external plugins that I have ...