Still should be the tranny seal. That seal is what isolates the trans fluid from the adapter section. As in, regardless of how bad the t-case seals are, you shouldnt see tranny fluid in there. Hope that makes sense. It will make more sense when you get it down and look at it.
Usually it leaks at the adapter bolts as well, but if its getting into the t-case, the t-case input may need a seal as well.
I’d get both while you have things out. Should only be about 6$ each.
The tranny one is literally 30 seconds to do once you have the t-case off. It could have been pinched when you put the t-case in, or just a defective seal.