I know the embedded bridged UI comes with a bit of a disclaimer around reliability, but I thought I'd mention anyway in case there's an easy fix:
When I toggle windows back to Reaper, sometimes the Kontakt windows display a gray window and don't redraw until I interact with it. If it's possible, it may be an easy fix to force-redraw the plugin window when Reaper takes focus.
Reaper 6.14 on Windows 10 x64.