Exporting 1080p from 4K Timeline, All non-4K content is fuzzy (23.3 and 23.4)

I've got a project that was shot in 4K, and requires both 4K and 1080p deliverables. I'm working in a 4K timeline. Though the project is 95% 4K, there are a couple low-res stills or 1080p clips from other sources that need to be dropped in and scaled up (using set to frame size).

This series of videos have been a years-long process so I've been through several at this point and here's how it's gone historically (aka Premiere 23.2 and before)

4K Export (CUDA-on) - GOOD! 1080 and lower quality images are stretched, but they look like one would expect, not great, but good enough for our purposes

1080 Export (CUDA-on) - BAD! 1080p and lower quality images look extra terrible. Like a 640x640 jpeg might come out looking like a 100x100, wayyy worse than it looks in the timeline

1080 Export w/ Max Render Quality checked (CUDA-on) - GOOD! 1080p and lower quality images look "good enough" as expected

I just make sure to have "max quality" checked and everything's been great until 23.3. After that update (persisting in 23.4). Now the results are like this:

4K Export (CUDA-on)  - GOOD! 1080 and lower quality images are stretched, but they look like one would expect, not great, but good enough for our purposes

1080 Export (CUDA-on) - BAD! 1080p and lower quality images look extra terrible. Like a 640x640 jpeg might come out looking like a 100x100, wayyy worse than it looks in the timeline

1080 Export w/ Max Render Quality checked (CUDA-on) - BAD! 1080p and lower quality images look extra terrible. Like a 640x640 jpeg might come out looking like a 100x100, wayyy worse than it looks in the timeline.

1080 Export w/ Max Render Quality checked (CUDA-off) - GOOD! 1080p and lower quality images look "good enough" as expected (but takes forever of course)

For now, it's been faster for me to export the 4K copy with CUDA and then transcode that to 1080p than it is to do a non-CUDA export of the 1080 copy. That's well and good for now, but I want my 23.2-and-before workflow back!

I've tried rolling back my Nvidia driver to the previous studio version, no luck there. The PC is a Puget machine with a 3090, so it should be giving Premiere what it needs. The issue persists even if I start in a clean project. I've also tried nesting the timeline in a 1080p timeline, that didn't work either. Further, it happens directly from Premiere or in Media Encoder. For now, I'm jumping back to 23.2, but I've got other projects in 23.4 so I'm constantly going back and forth. 

Any thoughts are appreciated!