The TL:DR version - the problem may go beyond artwork not syncing; all music syncing may be screwed up.
I have the "missing artwork" problem. I have three devices, and learned that the problem occurs when you sync with iTunes. My iPad was the only device that still had artwork - and I rarely sync it to iTunes. So I plugged it in and did a sync - bam! Artwork gone.
I tried a bunch of fixes on my Mac; permission fixing, disk repair, etc. I even moved my entire library from my external hard drive back onto my internal drive; no dice.
I also noticed that I could no longer retrieve artwork in iTunes in certain cases; bought Veruca Salt's new CD "Ghost Notes" and got an error when trying to retrieve the artwork after ripping it. As a test, I have a Mac Mini that I don't use iTunes on (but had done the 12.2 update on). I copied my entire music library to the mini and then did an artwork retrieval; I got a lot of artwork, but not for "Ghost Notes". I checked around and saw several other cases were I couldn't get artwork, even though the album was available on iTunes, and I had been able to get it in the past.
Even worse, I tried to sync "Ghost Notes" to all three of my devices; it wouldn't go (the songs never transferred, and "waiting for changes" took forever). Eventually I wiped my music library on my test device (this time, an iPhone 4s) by disabling sync. For some reason, my purchased music stayed on the device (all of which had their artwork), so I deleted them manually from the phone. I then copied over seven Veruca Salt albums; with the exception of "Ghost Notes" they all have their artwork.
Funny thing; I can still sync TV shows and apps, no problem.