Media library greyed out after migrateguru migration

I just did a migration with Migrateguru to move my client’s site from wordpress.com to a self hosted domain on IONOS. All plugins are up-to-date, I’m using WordPress 6.02 and PHP 7.4.

From the front end, everything looks fine. All text and images look the same as they did on the site I was cloning. However, on the backend, the first 165 images in the media library are just showing as greyed out squares in grid view. The rest of the images in the media library seem to be OK, but I haven’t yet tried to look at all 6717 of them. In the list view, there are thumbnails displayed for all images, including those first 165.

The 165 images that are greyed out correspond to the most recent three months. In other words, they are for every image in the 07, 08 and 09 subfolders of 2022 and they are a mix of .png, .jpg and .webp.

In most cases, but not all, if I click on one of the greyed out boxes, I get a full size image that looks fine. As an experiment, I tried editing one of the images by doing a vertical flip, twice, to return it to it’s original orientation. After that, the image icon was displayed correctly in place of the greyed out square, so I’m pretty sure it’s a problem with thumbnail for the image not being present, or somehow not matching up with what the database expects.

After doing that experiment, I looked in the 09 folder where the image file is and found that there are now now 9 versions of this image, including eight that were just generated as various size thumbnails from 150×150 up to 400×400. The original image was 601×600. I tried the same experiment on one of the images that didn’t show up when I clicked on the greyed out box. When I clicked Edit image, it did show me the image, then after doing the double flip and saving, both the icon, and the larger image were shown, but there are now 21 versions of this image in the 09 folder. The original image size was 3278×1645, so there were lots of smaller thumbnail possibilities. (I really need to talk to my client about not uploading full size images)

I know I can fix this by using the Force Regenerate Thumbnails plugin, but if I do that, I’ll wind up with a whole lot of thumbnail images that I probably don’t need and the already huge media library will grow substantially. I’d like to understand why this happened in the first place and find a more elegant solution than brute force regeneration of all the thumbnails.

The page I need help with: [log in to see the link]