Not sure if this is a Reaper-only bug, but just in case, I am using Reaper 6.57 on Windows 10.
When I first load the KORG MS-20 (either VST2 or VST3), the GUI is correct.
If I save my Reaper project and re-open it, the KORG MS-20 GUI opens with the sequencer displayed, even though the "Hardware" button is highlighted.
If I click on the "Hardware" button, the sequencer controls stay visible. I need to click on the "Sequencer" or "Effects" button and then click on the "Hardware" button to make it display only the default hardware section of the GUI.
I wonder if anybody else (Reaper or otherwise) is having the same issue.
In case it helps, I attached some screenshots. Note that in the second one, I highlighted in red where it says it is displaying the "Hardware" section (not the "Sequencer" section), but the sequencer controls are displayed.
It appears that the application is unaware that the sequencer is displayed, because if I run a tutorial, the sequencer stays open and the tutorial highlights sections of the sequencer when it says it is highlighting sections of the hardware controls. If I intentionally open the sequencer and run a tutorial, it closes the sequencer when the tutorial starts.