Carheir,
I have a B2650 which is pretty much the same tractor, and have not had a problem with the fill cap. Could it be a defective cap? Certainly could be. Could there be a bad run of that part and it will be a problem for others as time goes by? Certainly could be. Maybe not likely, but possible. If it pops off again, you will have a good indication of the problem.
In the meantime, let's see if any others report a similar issue, and you can go from there.
For your own information, I have gotten "jabbed" once or twice by what I thought were curt answers. Some of the really knowledgeable members can be that way sometimes. I guess that the best policy for this forum is to not say anything you wouldn't say if your wife and kids were listening.
Good luck to you with your Kubota. I am sure this small problem will be resolved, and you will really enjoy your "Orange Beast".