I ran Disk Utility's First Aid on my drive's APFS container. One of the volumes, called "VM", gives the following warning:
warning: Overallocation Detected on Main device: (26707978+1) bitmap address (3bc0d)
I also ran
diskutil verifyVolume /dev/disk1s4, where
disk1s4 is the identifier that corresponds to the "VM" volume. This did not return a warning. With that in mind, and knowing what the VM volume is for, I suppose this case is not a problem, but a consequence of running
verifyVolume on the entire container.
That is, since the VM volume is used to swap between volumes, it is probably used during the process of verifying the container, and that's why it returns the overallocation warning. But that's just my speculation.
This one thread says that reformatting the drive was their solution. However, I just reformatted my drive yesterday and had to wait a grueling 12 hours (I timed it) to restore all 700+ GB from Time Machine. That thread had many other system file problems, so I hope my case won't have to resort to something as drastic as starting over from scratch.
I appreciate your insights.
PS: In case you're wondering what the VM volume is for, here is a link.
PPS: I'd rather post this in SO, because that site has more hits for the keyword search "allocation." But I got sent here instead.