Question about Facebook tracking

I was on a Facebook group reading some posts. The group settings are set such that people who have seen the posts are made public for all to see. As a test for Freedome VPN that I just downloaded i looked at a post from this group to see if my name would be logged as having seen the post. I was surprised to see that while I freedome VPN was on. My name was still logged as having viewed the post. I thought VPN would prevent this.