Thanks for the input. In regards to anti-skate settings, I've taken screenshots of some various settings. These are all line-level captures from the tape-out of my Onkyo A-9555 with no gain applied. I see the same behavior regardless of cartridge/headshell used as well.
A standard new LP with my normal anti-skate setting of 1.75g:
12" single with normal anti-skate setting of 1.75g:
12" single with 0g anti-skate:
12" single with 2.75g anti-skate:
12" single with 3.75g anti-skate:
My anti-skate definitely works as evidenced by how the arm behaves on a single-sided record. I think the photos above should show why I was wondering if this was not something to do with how these singles were mastered or cut.
Another thing I would point out is that when I listen to these records, either the original vinyl or my digitized copy, I don't detect that the left channel sounds louder or that there is a channel imbalance. Imaging seems like it is pretty much what it should be, it just looks like there is noticeably more stuff in the left channel.