what about a transmission case vent?
some have them
if the vent gets clogged up it can allow the transmission to build up pressure which might explain why the fluid is getting pushed into the hydraulic system.

Generally by design seals are designed to keep fluid inside their contained systems not the other way around even though the dust lip on the seal may help prevent any dirt from entering in from the outside liquid tends to go where it wants to. So this in theory could be what is happening.

Have you ever pulled the dipstick out of the transmission and had it release pressure? Or have it keep blowing the dipstick out? Or if it has a capscrew plug with a filter element inside it release pressure when you removed it?

check for a transmission vent either on top of the case around the control valve area or on one of the axle tubes and even possibly inside the dipstick cap/****. I'm betting its on top of the trans case though.