Not a Bug - use Full Precision UVs with large offsets
When a user sets up an unwrap using a space outside the original 0 to 1 space the unwrap will increasingly show errors of positioning. For Example, in the following images the lower left vertex of the plane is +0.0001 units, lower right vertex is at -0.0001 units. As the Unwrap moves away from the original 0 to 1 space, the small texture wrap increasing grows.
Also Reproduced in Main Promotable-CL-2564509 & Releases/4.8/Promoted-CL-2560772
(Instructions specific for Maya but was also reproduced in 3DS Max)
RESULTS: UV_Test_Original.fbx and UV_Test_UV20.fbx despite being in the same 1 to 1 environment there is a huge difference in that 0.0001 vertex movement.
EXPECTED: There should be no difference in the unwraps.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-16222 in the post.