When I replaced the filters, the old ones were wet. That's gotta make a difference. Even I know that.
You need to find out why the filters were wet in the first place. There is no reason for moisture getting into the return ductwork.
There must be some air leakage in the return side for water to get into the system.
Sort of chicken and the egg. I'm sure the coils freezed up, and when they thawed, they dripped down to the filters. Now why they froze up in the first place, that's what I don't know.