Morning everyone!
I’m sure you are spot on about DPI scaling causing the problem. It has been some time since I’d looked at the MadVR HUD, had been looking at some 2.35:1 movies and had assumed without doing the calculations the 1536x864 numbers were due to letterboxing. Now I realise they should show the entire viewport size and are in fact exactly 4K/2.5. I am using 250% DPI scaling.
I didn't look closely enough when using Life of PI, so didn't see that too had the odd numbers.
I didn’t originally have display mode switching set up in MavVR, only in MC Video Settings, but I added the 2160p modes to MadVR early on to try to fix. I thought maybe the MadVR settings had got lost in the move to MC23.
I tried changing Windows desktop mode down and back to see if that would fix but it made no difference. As I said earlier, I actually change the MC display mode right down to 720p and it did nothing. Still the refresh rate obeyed the rules set in MC but the resolution remained 4K/2.5. It seems MC/MadVR was stubbornly using 4K/DPI Resolution as it's target.
Anyway, after making all these changes and completely deleting MadVR and it's settings at one point and letting MC re-add them things hadn't changed. Reapplying my display settings didn't help either, but a reboot after all this has completely fixed things.
So maybe this is one of these completely random things that will never be explained, but for some reason MadVR was using DPI Scaling to calculate it's viewport. It might be worth looking into how conceivably this might happen, particularly as people might not notice right away, if this 'bug' (for want of a better word) crops up, unless they happen to use the HUD.