Create a Material that uses a Static Switch Parameter to optionally use a Vector Parameter.
Create Material Instance. In the Material Instance Editor, use the Static Switch Parameter to enable the Vector Parameter, and expand its dropdown.
Use the Static Switch Parameter to disable the Vector Parameter. Observe that the dropdown disappears.
Repeat steps 1-3 above but with the parameters as part of a material layer. Observe that the dropdown for the Vector Parameter does not disappear when the name and color selection box for the Vector Parameter disappear.
Have Comments or More Details?
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-193624 in the post.