Confirmed that this is still a problem as at 1.8.8 and it doesn't only happen with a single trade. It happens with a villager who has multiple trades. They can all be locked out with no way to reset them, and it has nothing to do with unloading the chunk since it has happened to me when I've deliberately waited for the villager to reset his trades.
Why doesn't the game logic simply check if all available trades are locked out and if so, reset all trades upon the player exiting the trading screen?
Confirmed this is still happening as at 1.8.8
Confirmed that this is still happening up to 1.8.8, at least for me.
It's still happening in 1.8.8 and it's not only with baby animals either. I've seen an adult sheep walk straight through a solid fence. Yes, I saw the bug as it happened, so I know it's real and it's not just an issue with animals growing up.
I think that sometimes, animals suffocate when they try to pass through some blocks. I've found dropped passive mob remains next to large jungle trees while in peaceful mode.