Ah, bingo: https://github.com/LemmyNet/lemmy/issues/3722
I may have bumped into a bug while testing comment functionality.
I think those two other replies were comments that I deleted. I assume the cause of the bug is comment 2566997 having a child_count value of 2.
child_count
Ah, bingo: https://github.com/LemmyNet/lemmy/issues/3722