No Saved Album Artwork

Happens on 3.17 and 3.18 (yeah, just found it on 17, and upgrade to 18 didn't fix it). After pasting album artwork, and saving it, thumbnails on Windows10 doesn't show it, nor does it appear under Artwork on iTunes. Appears in MP3Tag just fine.

Looks like a player problem.
You have to update iTunes somehow - but this is not in the scope of MP3tag functions.

What kind of files have you treated?

Regular MP3 files. Both Windows Explorer and iTunes don't show it. So it seems to be a saving issue. Not saving whatever tells the programs to display it. Even VLC is blank and that always shows artwork.

Please see the hints for support requests and tell us what the result of the various steps to analyze possible causes is:

OK. Problem is any artwork attached to an MP3 file and saved in MP3Tag 3.18 is not viewable in Windows Explorer for Win10, nor VLC, nor iTunes. Not a problem in MP3Tag 3.16
InMP3Tag Original:

In MP3tag Edited- The Cover and info are now in the picture box itself):

In Windows Explorer:
In iTunes:

Does it make a difference if you change the picture type from "other" to "front"?

OK, what you contributed is the part of the screendumps.
What did the examination with the linked tools reveal?

Already on Front. But same thing with Other.
MP3Diags Tool says:

"ID3V2 tag has an APIC frame (which is used to store images), but the image couldn't be loaded.
Error loading image in APIC frame."
So the problem seems to be APIC Frame

Looks like the picture that you use has an invalid format.
So try to get a picture that works.
First remove the old picture and check the files with the tools to see that everything is OK

OK, seem to have found the problem. I can no longer copy an image and paste it into MP3Tag (Right click on image box, selecting Paste). If I copy the pic, paste it into an image program, save it, and then use add image in the MP3Tag image box, it saves fine.

This looks to me as if the image you copied to the clipboard has an invalid format and when you save this image with your image program you save it to a valid format. Mp3Tag allows JPG and PNG.
Your problem does not occur here with pasting an image.

What was the source of the image? Many web pages are now using webp as an image type. Most players will not recognize these even if they are embedded. For best compatibility use either jpg (not progressive) or png types.

That might be doing it. Might have been webp.

The screenshots that you supplied show "jpeg" as format.
Also, MP3tag supports webp and has an action/function to transform that into jpeg or png.

Yet, it looks to me as though the picture was damaged. MP3tag does not evaluate a picture's integrity - so if it comes damaged from the source, it will be embedded like that.