Fed two chickens some seeds. Both showed hearts. But they didn't actually more towards one another even though they were within horizontal range. (Initial vertical range was 3-4 blocks, I estimate.) Luring them together with seeds didn't help. Once they were standing practically in the same block, I switched away from seeds to some other hotbar item, but they still ignored one another. See image; they're adjacent, they're both emitting hearts, and yet they're still not getting it on
After the usual delay the hearts timed out for both, I re-fed them and they bred OK on the second attempt. I've never seen this breeding glitch before.