Alright, just popped in game and was able to confirm it is this "ground type" issue I mentioned. Long story short there are two types of "harvest ready" ground types and for some reason AI mowers (which swathers are technically a mower) will not acknowledge the "_OTHER" ground type. I noticed this when I was making an update for Swathing Plus and assumed it as unique to sorghum and rice since those do not natively support swathing. This looks like a game bug to me so I will report it and see what Giants says.
If you are on PC you can force the ground type to be the regular "HARVEST_READY" using Easy Dev or Power Tools and it will allow AI to work.