I am on Riverbend Springs and dont have the DLC or any other swathing mods. I found it really weird. I have tested against Wheat, Barley, and Soybeans, and they all work fine with the AI Workers, its only Canola that the AI just refuses to do.
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.