Load up a registration that is causing the buttons to change. Then, re-save it with the 'assignable buttons' box unchecked.
To check that it worked. Go to a different registration. Set the assignable button as you want, and then load the registration you just saved. It should now stay the same.
Rinse and repeat for each Registration that is causing the issue.
Just un-checking the button isn't enough. You need to re-save the registration with it un-checked.
(And don't forget to save the Registration bank file)
Adam