This also happened on the Iowa Plains View map, just last week. It also happened with straw swaths on that map. Will test a grass field today on this map
Viewing post in FS25 Judith Plains, Montana 4X comments
I just started a new save this morning on Judith Plains. Mowed some grass into windrows, then ran a baler over the windrow. It worked perfectly, so not sure if your issue was related to something else. But like I said, I had this exact same issue on Iowa Plains View for both grass and straw swaths that were laid down prior to the patch.