Are you sure that each and every track has null for that custom tag?
Well, well, well. There are two tracks on that album that do not have a link in the tag. Once I added a test link, the highlight showed up on the album artwork.
I had -thought- that I had albums with links on -some- tracks and those albums displayed the highlight, but it further testing I've determined that not to be the case. Doing a custom search for "link has a value" and displaying the results as album art view, every cover shows the highlight. But that's because each album is only showing the tracks where "link has a value."
My "solution" for getting any album that has at least one track with a link, is to tag the tracks without links with "
www.dummylink.com." I have 100 albums with at least one link and I don't have the patience to tackle them all at once, but as I come across them, I'll add the dummy link.
Thanks.