Blurry images/video in EXPORT and PLAYBACK -- this was not happening before....?

I cannot figure this out for the life of me. I completed a project recently (about a month ago) and exported it as a mp4 video with "Match Sequence Settings" which I always do (4K footage in project Sequence settings). The video looked perfectly fine.

Now, one month later, I re-visited the project to replace a single audio file...and I see that certain video clips and image files (.png) in the project appear BLURRY. In fact, it's video clips and images that are zoomed-in or scaled-up. They appear blurry in the preview playback as well as in the export. This was not happening before. I noticed this after I tried to export the project again (after replacing the audio file) and I noticed that the newly exported video is blurry in those specific parts. I can literally do a side by side comparison between my old exported video and the newly exported video and see there is a STARK difference between the two videos, even through they are exactly the same!

But first look at this: when I hit Export Media and get to the export settings...

(left is MATCH SEQUENCE SETTINGS and right is not matched but H.264)

The left one is blurry when I click "match sequence settings", but, if I unlick "match sequence settings" and set it to H.264, it is perfectly sharp.  Why??? Again, the above clip is a zoomed-in clip of the source video file. This also happens to other zoomed-in images and videos. Other images and videos that are not zoomed-in appear fine. But let me say again--this was never happening just a month ago!! All of the images and videos, even the zoomed-in ones, were perfectly sharp when I exported and when I preview-played it just a month ago. Clearly they are capable of being sharp--when I set it H.264 they are sharp! And my old exported video is still perfectly sharp...I am utterly baffled.

Note: I do NOT want to export as H.264. This degrades the image in the rest of the project. I tried exporting the above project as H.264 and the footage is very clearly degraded compared to the original exported video in matched sequence settings. I have always matched to sequence for export and it has always come out perfectly, never any blurriness.

By the way, these are my sequence settings for this project. I have no idea what happened in the past month, but SOMETHING happened or something was changed that is now, suddenly, causing resolution to be blurry. But I have no idea. I do recall changing ANYTHING....!