Some specifications have internal usage or scenarios that need to be hidden, but the store needs that specification for purposes of the search, like using it for Merch Rules.

On some accounts, the SKU specification "color" does not appear as an option under "Use SKU specifications to display individual products in search results". We see that "color" is one of the specifications under "Hide facets".

Remove the attribute from the "hide facets" configuration, create the merchandising rule or do the expected configuration, and hide the attribute again.