Cows suddenly won’t go through Nether portal

I brought Mooshroom cows through a Nether portal, across the Nether, to a different portal. This portal still functions (I can go back and forth without issue), is longstanding, and had never broken. I’ve tried exiting and rejoining, and restarting, but they won’t go teleport back to the Overworld. When I return from the Overworld, I’m automatically still attached to their leads. I have brought many mobs through this portal without issue.