Disk utility says partition is unformatted, Fsck says Macintosh HD is OK
Solution 1:
Thank you to @David Anderson for the solution.
I think the problem was that when I was modifying my partitions to make space for Pop!_OS, GParted changed the GPT partition type. I didn't notice at the time as GParted identifies partitions by their filesystem signature as opposed to the partition type. I used gdisk on Pop!_OS to change the partition type from AF05 to AF00 and it boots properly. Yay!