I assume you mean 2160, and not 21160.
If I do resolution divided by 3 (300%), then I get a virtual resolution of 1280x720 for your device – Roon cannot run on 1280x720 – and the fact that you have a taskbar fixed at the bottom, means you are actually getting less than 720 tall.
Fullscreen mode works because draws over the taskbar, and hides the top statusbar too, so you get exactly 720 tall, which is just barely enough.
The good news is that your options are simple and plentiful:
- change your scale factor in Windows
- change the scale factor in Roon with a command-line parameter (-scalefactor=2)
- you can run fullscreen.
The link @mike posted above has instructions on how to do both of the first 2 solutions.