Can an outlet become un-grounded over time?

Short answer to a direct question in court. Yes.

Longer explanation as to degrees, and how it could happen. – If the original wiring was not properly “snugged” up to the receptacle, over time and use it could detach from the screw.

Or, if the ground wire was damaged “upstream” from the receptacle it might have broken. A broken ground wire thus ends the path to ground for all devices “downstream.”

Another possibility, depending on the age of the receptacle used, and if it was used in a manufactured home, is that it may have had a design that did not physically attach the ground wire to the receptacle, but merely wedged the receptacle to a ground wire in the box. This type of device and wiring method was stopped in the 1970s.