I recently purchased round bales from a different farmer. These round bales are clearly horse crack type hay, aka it has some alfalfa in it, versus the straight timothy that the square bales in the barn are (and their previous round bales were).
This whole week (first new round bale) my horses come in at night and look at their square bale hay and give me the evil stink eye. They are eating about 1/4 of the hay they normally would in their stalls over night. (But running out to that round bale every morning like a kid on Christmas morning running to their presents.)
Using your theory suddenly the hay that has been fine all along is now bad hay and I should just toss it out.
Horses, like dogs, cats, kids, husbands, etc will not eat their boring food if they know there is an option for eating something better.
I like the idea of giving them a few days of very little extra yummy hay and mainly the regular hay and see if they start eating it. I bet they will.
Walker, rained on does not equal moldy. If the hay is properly dried prior to baling and does not mold in the field it is just fine. Clearly some hay rain = no chance of it not molding when things are just not right to be able to get it dry. But a blanket statement that rained on hay is always moldy is not accurate at all.