Static mesh LODs other than LOD0 cannot be set to screen sizes greater than 1
From this AnswerHub thread:
Open a static mesh with LODs (or generate some)
Disable auto calculation of screen sizes
Try to set the screen size of a LOD other than LOD0 to a value greater than 1
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-41125 in the post.