Update:
Success at last!
So, it turns out that the VST2 version can’t receive PC midi messages, it can only receive note value and CC messages. I switched back to the VST3 format after an hour of mental pain, and it finally works perfectly like in the standalone! (The problem was mainly that I didn’t know how to configure studio one for midi control properly, nothing was wrong with the Plini vst3).
Big thanks to your detailed setup info and suggestions! Everything’s good now 