96 Trooper - AT Fluid blowoff

Hello -
Got the AT fluid flushed and re-filled about 18 months ago, and now whenever it's relatively hot (> +80 F) and we drive for over an hour or so, the fluid
blows off down the right side. There is a metal tube coming up from the transmission (passenger side of the engine) where it then goes into a flexible rubber tube and back into the fender and terminates there. The fluid runs out of the wheel well and then blows down the side of the truck.
The garage I had it done at says it may have been slightly overfilled and when it reaches a certain pressure it blows off.
Does this sound normal? Anyone else seen this?
I believe this is the TH400 transmission - does anyone know if the blowoff tube is standard on these transmissions, and if so maybe I could take it to a GM dealer for a closer look?

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