Create a material that has a ChannelMaskParamater in it.
Create a Material Instance of this material, and observe that there are two checkboxes for the parameter, one with a selector and the other with a vector option.
Repeat the above, but have the parameter be part of a Material Layer. Observe that the Material Instance shows only one checkbox and it has both the dropdown and vector options.
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There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-193622 in the post.