Meanwhile, this issue has been taken up in different discussions on this site. Apple has still not acknowledged the issue and keeps recommending to Repair the library, (cf. the first answer by Joseph_S. on Nov 17, 2019 7:27 AM), which is bogus and which tacitly admits that they have no solution, and there may be irrepairable damage due to a serious bug by Apple.
I have analyzed this error, which is characterized by the following behaviors: (1) photo is shown in lower quality than the original, (2) editing is impossible and produces the well-known error “Photos cannot load adjustments for this image”, (3) it is not possible to reset the adjustments, (4) editing in any other application produces an error, and (5) exporting the unmodified original is not possible as it produces an “Unknown error (4)”. Repairing the library reports success but does in fact NOT solve the problem. I have also examined the place where Photos keeps originals and the files for the affected pictures are NOT THERE ANYMORE. From these behaviors it seems that Photos has lost the original and the library is left with a low-resolution preview. Not sure why and when this bug started manifesting itself, but I noticed it first in Mojave and it persists in Catalina. It seems to affect some photos from 2015. I have not found any working solution and it is unlike that a fix can ever be found, since indeed the originals of the affected pictures cannot be found in the photoslibrary. They have simply vanished. It’s time to hold Apple accountable.
The above was from the original poster on January 20, 2020.
I wanted to check if my originals are still in iCloud. They are not. iCloud has nothing but low resolution photos. Now with 26,000 photos I obviously cannot check every photo. But I checked 8 photos from my trip to Vienna in November 2019. These are photos I cropped and know for a fact are sharply in focus. No high resolution originals to be found.
I called AppleCare. They do not support public betas. Therefore, even if the building is on fire, they cannot call the Fire Department. Their answer was “Well, that’s the risk you take using beta software.” Their only suggestion was to send Feedback which I did already.
This is a developing story.