This happens because the Collections module isn't loaded when the FFrontendFilter_TextFilterExpressionContext instance is created:
In addition, this check in the FFrontendFilter_TextFilterExpressionContext should not really include the bool, as the AssetCollectionNames list is used by the explicit Collection and Tag keys, and those should work even if bIncludeCollectionNames is false, as that bool is only supposed to control what simple searches test against.
Expected: There are results.
Workaround: Close and re-open the Content Browser.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-70917 in the post.