I don't know Grandpa's farm, but I have noticed that the modded farm maps can be a bit tricky to leave. Have you tried different routes to leave etc.? For example with the farm map I'm using now I've noticed that if I go North, I have to keep to the left or the character get's stuck being able to only go east and west. The bottom exit seems to work normally, but if I exit to the east, I'd better be to the right (south), if I don't want to tinker to get my character to the busstop area.
So try to test: the actual warp area may be only part of what it seems to be.