This only seems to happen when the vector parameter is expanded, not when it's collapsed. Collapsing and re-expanding the parameter seems to clear the issue as well.
The material seems to use the override value when the checkbox is checked, too.
Alternately, see attached project for repro. Open M_MyLayeredMaterial_inst and expand the vector parameters of
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-108014 in the post.