1080p or 4k: either or, but not both!

Hi,

I have an issue that keeps coming back to haunt me, it always seems fixed but then it returns. Hoping y'all are able to help me 🙏

Background:
- OS: OSMC
- Hardware: Rapberry PI 4B (4GB)
- TV: Samsung Q60t (65" 4k)

Issue:
a) I see stuttering in 1080p playback, which I can kinda solve by whitelisting the 1080p resolution/framerates.
b) if I do the above, I break high-bitrate 4k playback, and I have no idea why!

When either works, they work flawlessly. Full size blueray remuxes with HDR + Atmos are fine without the pi breaking much of a sweat.

I have played the same file I've used in the logs below recently, without any problems. Same situation with other known good material.

Here is a solid debug info dump, including:
- debug logs from a garbled 4k playback
- modeinfo
- mediainfo for the 4k file
- kodi settings
- a config diff that broke 4k but made all 1080p stutter free

Any help would be hugely appreciated 🙏 Let me know if there is any other information that is needed to get to the bottom of this!