Hello everyone! I hope one of you here can help me (us) with this problem.
So the problem is pretty simple to describe, if the material is applied in the static mesh editor the material looks off. (I will attach a video instead of trying to describe it) If the material is applied into the material slots that are only available after a mesh is dropped into the scene, everything seems to be fine.
I have done a lot of trial and error stuff in the last days and worked together with a fellow artist.
Here are a few things we found:
The problem is not only on my end but is reproducible by others.
The problem seems to only occur with Speedtree (version 9.2.5 and 9.0.0. tested) models as far as we tested it.
Different texture setups and export methods (from Speedtree) did not help.
We exported the models in different formats (obj, fbx, st9, st, usd) nothing changed.
We tested a lot of different material setups in unreal the problem always was the same.
(Just a rough summary of what we have tested so far)
Thanks in advance to anyone how looks into this or maybe even knows what kind of stupid mistake I made. 
Video of the problem: