I think the issue here is, when checking if the component has tag (not the actor tag), the component tag is not correctly detected. This must surely be a bug in UE4, not a design thing.
Im having the exact same problem. Im giving my static mesh component a tag of its own, and when I check for the tag of this component on a “OnComponentBeginOverlap” event, it gives me null.
Bug persists in 4.16