Notice the wings showing through to the other side. I’ve already checked in blender, and the face orientation appears correct (blue on all sides), and the scale is not negative. Changing the material’s blend mode to “masked” seems to fix the issue, but the lighting looks terrible.
I’m using the material created by Unreal by default when importing the model.
When the render mode is “translucent” you don’t get Z buffering for separate translucent triangles within the object. This is why “masked” works – with “masked” mode, you do get Z buffering.
Given that this object has almost no transparency, you should make most of the triangles not have a translucent material.
The best way to tech this object, for any game engine, would be to split the object in two subgroups; one has triangles that are opaque, and uses an opaque material (this would include the body,) and one that has triangles that are translucent, which would only include the feather fringes.
This is not Unreal specific, every game engine has pretty much the same requirements, and understanding and building assets based on these requirements is one thing that makes a “game artist” different from a “film artist.”