I’m a bit surprised that whitelisting 1080p modes would cause problems - that might be a bug. But it’s expected behaviour that your video is downscaled if the whitelist is empty. With nothing whitelisted, Kodi is able to switch resolution upward, but only if the video dimensions exactly match one of the available output modes. So, it can switch from 1080p to 2160p if the video is 3840x2160, but if the video is something like 3838x1604 then it can’t. You would definitely need to whitelist 2160p@23.98Hz to get it to play at 4K.
Whitelist guide: (click here)
If the whitelist doesn’t seem to be working the way it’s supposed to, that would be interesting to know about.