@jpedty I've not been able to recreate this problem on my AC68U. What model router do you have? Maybe it's specific to the N66U/AC66U.
EDIT: You should probably have done a factory reset after you noticed the initial corruption.
When I installed 33E7, I did a full reset. Though I did restore my config using the nvram restore scripts rather than key everything in again.
I didn't encounter the user/password corruption mentioned by jpedty. Subsequent up/downgrades back and forth between 33E and 35E, I haven't reset nvram.