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.
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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.