MC 28.0.66:
Select all tracks in an album. Open Tag window. Click "Remove Cover Art". Wait tag changes to be written to files...
If you did this in a library view MC shows that tracks no longer have cover art. Cover art is however still inside the file. I checked this in two different ways: 1. using HxD (hex file editor) and 2. using MC: i.e. drag & drop the files to Player window and select the files and MC shows that cover art is inside files.
Select files. "Remove Cover Art" then "Add From File...". Replace with a totally different image. New cover art is shown. However it has not been written to a file. Drag & drop the files to Player window. Original cover art is displayed not a new one.
The problem is that I cannot change cover art using MC anymore. I have Sony DAP which cannot display progressive jpeg images (thank you Sony!) so I used to have a workflow: extract cover art, use XnConvert to convert progressive jpeg image to baseline jpeg image and use MC to change the cover art.
I have tried "Also store image in the file's tag" disabled and enabled, "Do you also want to permanently delete the cover art files from your hard disk?" with "Yes" and "No", "Clean Cover Art Folder..." and all different options under "Audio mode:".