Oh, I apologize for being unclear! Let me clarify:
One of our regular users used the block feature to block a different user. The user that they blocked, however, posted to a group. Posts to the group then go to the "everyone" feed, so even though that user blocked them, they are fully seeing that user's posts because they are to a group.
Put another way, it's like this:
User A blocks User B
User B is in Group 1
User B makes a post in Group 1
Group 1's posts appear on the "Everyone" Feed
User A is then seeing User B, who they blocked, because User B posted to Group 1