BUG - Translucent materials display incorrectly in material preview:
Here is my mesh in game:
And here is the material preview for the said mesh:
Hey marcomaryred -
Is the M_Glass material included as starter content also previewing incorrectly in the Material Editor for you?
I will need to see what your material layout is to help reproduce this issue.
Thank You
Eric Ketchum
Just tried the M_Glass provided and this is how it looks in engine and in game:
still looks somewhat broken but I can post my material if you still need it ! 
Hey marcomaryred -
Part of the difference in the Material Preview and the in Engine Rendering difference is the added reflection and refraction information that is added by the engine. the lighting intensity and reflection cubemap is not shown in the material previewer. There is also the issue of opacity which will fade the base color in the previewer as a means of showing the translucency.
Hopefully that makes sense, but I would be happy to continue to research this issue for you.
Let me know -
Eric Ketchum
Actually that makes perfect sense, just a bit of a hassle really, nothing more Thank you for your reply !