1994 Lincoln Continental I had a leak and got it fixed but

I've seen this happen with lincolns with higher mileage.
The internal parts wear, but the weakened viscosity of the fluid no longer is protecting the transmission. While it works with the old fluid, it is a matter of time before it goes. Introduction of new fluid can accelerate the failure of worn parts. Sometimes immediately and sometime shortly after.

It probably needs a rebuild. I would not fault the mechanic on this one. He is doing what probably should have been done 4-5 times by now.