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 ...
FLStudio for years now.
Now I have changed my workflow to work with an Akai MPC.
The MPC can load VSTs and I have installed the Juicepack and the Maximus. Both are licensed already.
I have tried it on two Notebook ...
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 ...