I was luring a sheep into my base with some wheat, and it couldn't get in until I removed the rails from this patch of dirt here (see screenshot). This isn't the only place where the sheep perceived the rails as an obstacle: getting them across a 1-block-wide railroad bridge was a nightmare, so this looks like a systematic thing. I get it that sheep are supposed to be stupid, but their locomophobia (is this a word?) in particular seems like a bug.