We have stored our hay on the concrete ground under an awning in our shop area for 15+ years and have never had one mold because it was on the ground. And that's ordering 200-300 bales at a time. Plastic can cause moisture, can it not? Maybe that was an issue. I'm no expert, though.
I've read so many people say the bottom of their hay does
mold, and so many say the bottom of their hay doesn't
mold. My take is that it's obvious there is no one hard and fast rule - it must be dependent on climate and local conditions - humidity of air, and if there's no vapor barrier, the dampness of the underlying soil, which will reflect amount of rain, soil type, and drainage patterns.
I can completely believe that some people have had no mold on the bottom of their hay. I can also state absolutely that my sister lost the whole bottom layer in a stall, on pallets over a concrete floor, with no vapor barrier (either under the concrete, or on top). Lesson learned. We took no chances when we stored hay in a stall; we put down several layers of vapor barrier (tarp plus well-overlapped huge bubble-wrap sheets from a local furniture/appliance store) and then pallets.
However, half my hay this year and all in the future (when we clear the loft) will be in the hayloft. We have limited flat and open buildable land here and I want use of my 2nd stall back. And housing animals is the least part of our barn's functions, as they live outside with run-in sheds. Being able to securely store a full winter's hay is #1. #2 is tack & feed storage, and #3 is indoor work space in inclement weather. So it's the correct choice for me.
I can say that my loft is definitely a dryer place than ground level in my barn. but with the ground prepared as mentioned above, we haven't lost a single bale to mold. And it certainly was easier to load into the stall than the loft.
All of which is to say, in lots of words, that there's no one hard and fast rule on what will work! I'd just suggest "better safe than sorry" if you don't know your local conditions.
The one thing I would add, which I haven't seen anyone else address, to the OP: don't believe "50-pound bales" are 50 pounds unless you weigh them! My "40-pound bales" are generally closer 30 pounds, and a few are as little as 25. (I take that into consideration when calculating that yes, it's still a good price, and yes, I got enough to last the winter.) I don't know if this is an issue in other areas, but one other HF poster (also in northern New England) also found her bales weren't quite as heavy as she thought.