Frothy trans fluid milkshake is water intrusion for sure.
I would first check the trans heat exchanger/cooler for leaks. It's the most common way for water to get into the trans if the bilge has not filled with water. Just because the cooler looks crack free doesn't mean that it is. You will need to have it pressure tested to be sure it's good. Most any radiator shop can do this for you.
Given your description of what happened last year you could have very well had water suck into the trans through the lower seal.
If you trans is a velvet drive it most likely does not have a filter.