All seemed to be working fine on the laptop (3060) so I decided to install MC 33 on the HTPC (3090), both using the latest 560.70 studio driver.
The new presentation mode (with the default 8 frames in advance) seems to be working fine (I've only tested 23p content in 4K119 OS HDR at this stage). I use Chapter 2 of Pacific Rim (UHD Bluray) as it's one of the most demanding content I know of, both for tonemapping and for frame drop testing as it's 16/9, so lots of content to process (no black bars).
Not only did I get no dropped frames, but I always get a repeated frame drop in this chapter on the HTPC at the same point, whether with JRVR or madVR, and for the first time with MC 33 and the new presentation mode I got 0 dropped, 0 repeated. Could be a one off, but it's encouraging.
So far so good, I'm going to test Atmos dropouts, I was still getting some with the latest MC 32 (build 58), so I was only using the HTPC in non bitstream mode, for non Atmos/DTS:X titles. I'll report back if MC 33 with the new presentation mode is any different.