Hello Midway,
I am not sure about a bug or not, but I have had problems with this sort of calibration issue in other areas of the games like in furballs. I noticed this and others had suggested that I use a powered usb hub for my joystick. I have since found that the issue was lessened by using an ordinary usb hub, but since it doesn't supply any power I still have this issue on occasion.
The POWERED hub is on my shopping list for next week so I can give you more feedback after that. The best that I could gather is that when the system hard drive, graphics cards and other input devices hog recources, the joystick calibration goes. I find that if I pull the plug on joystick go to auto-level and plug it back in, it will go back to normal. That is my temporary (on the fly) fix and sometimes it will take a couple attempts.
I know how annoying this can be, and if there are other causes known to any of you, please speak on them. I am all ears and about to invest in a POWERED usb hub that may not solve the issue even.