All album artwork gone on iPhone, aside from iTunes purchases?

I just noticed a mess-up in a song name, so, while I had my phone synced, I deleted the item on my phone, from the "on my device" tab on iTunes. I proceeded to add the correct version of the song onto my iPhone, and after doing that, along with the new song, it synced 27 songs which I had recently deleted. I then went to my music on my phone, and found that the entirety of my album artwork for songs that were not purchased on iTunes had been deleted! I resynced my phone multiple times, quit iTunes and reopened, quit the music app, and restated my phone; none of which worked. This is really getting on my nerves, and any light shed on this issue would be much appreciated.

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