Artwork changing when I transfer an audiobook to my iPhone

I have been manually (not via sync) transferring m4b audiobook files from my PC to my iPhone Max Pro 13 via iTunes. I am meticulous about having the correct artwork attached to my files, but about 75% of the time, the file that makes it to my phone has artwork that differs from what shows in my iTunes library, some random art taken from another piece of media on my phone. This is driving me insane, how do I stop this from happening?