I have now found a third album where the art does not appear. These are all FLAC with Vorbis tags. The art appears in WinAmp, Mp3tag and Tag&Rename. I have tried removing the art, and re-adding a different size, but for these albums the art never appears in VLC.
I have cleared the art cache. I have a batch file on my desktop with:
del C:\Users\donwi\AppData\Roaming\vlc\art\artistalbum\*.* /s /q
And periodically I go there and delete all the folders.
I have Windows 10 and the latest VLC version.