As of now, if you are following a tag you will only see the public content in your News Feed.
If you are member of a restricted community, you won't get any updates from that community when following the respective tag.
We have reported that issue and IBM answered: "I have received an update from our L3/Dev confirming that this is a case of working as designed - they will show public updates. Also as per documentation below - "When you follow a tag, updates are sent directly to your Home page whenever any public content with the tags that you are following is added or changed." Following Tags
I think we should be able to see all the content (if we are authorized to see) that is tagged with the tag that we are following.