Why are some photos badly pixilated when fully zoomed in?

TaP,

Does this sounds like your problem? It's been experienced by me, and certainly other, in what appears to be an exciting, all-new bug Apple has released for free to Photos users.

https://discussions.apple.com/thread/251495562

The common symptom seem to be that it's only the photos which have been edited that appear pixelated (or, to be more specific, appear to simply be the low-res thumbnail projecting as the full-size image). And, as soon as you click Edit, the actual full-size phone does appear. Which, to be sure, is as janky a hack as one could imagine. "Oh, yes, I'd love to show you the pictures from our trip, but I'll need to first switch into distracting, slow Edit mode, because Macs can no longer simply display photos in viewing mode."

Does this sound familiar, or do you instead have a different flavor of a similar bug? In which case, oooh, custom bugs! And who says Apple doesn't innovate anymore?

Download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Add Text button to include the report in your reply. How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Then we can examine the report and see if we can determine what might be causing the problem.

Welcome to the Apple community.

Could it be that you have optimisation turned on and that these photos are optimise photos that you haven't yet downloaded to your Mac?

Do the pixelated photos improve, if you wait a while after you have opened them in Photos?

Do you have the iCloud Library enabled in your Photos library?

Any photos can be optimised regardless of where they come from. What happens if you drag one of these photos to the desktop?

This seems to be resolving itself (over a long period of time) with some process evidently needing to complete in the background. The photos that were blurry before now appear clearly, but anything that I'm editing at the moment will only resolve as a pixelated, compressed mess when not viewed in Edit mode... until, presumably, some time in the unknown future. Maybe.

Still, another significant, vexing Photos bug... that no amount of running Etrecheck is likely going to fix.

I've been on Catalina since the ugly beginning, and the number of problems it has "fixed" for me is a massively negative number - I've never seen such a shoddy, awful OS release in the history of Apple.

You were smart to wait - it's obviously better sorted out, here, nine months after it was released. But I still experience more aggravation, slowdown, freezing, bugs now than in any time in my past decades of using Macs.

But if it fixed your problems, 👍 to that.

Etrecheck doesn't fix problems, it simply provides a snapshot of the system so those helping understand your set-up better. There is no evidence that your problem is wide spread or even vaguely common, if you are having a lot of issues as you appear to claim you are, perhaps you may need to ask wether your housekeeping is as robust as it ought to be.

OK, if you don't have iCloud photos enabled, they can't be optimised, so that's not the problem.