Reviewing through your description, the behavior might be expected as the "hide" feature should only prevent future activity from appearing. For example:
If I hide this specific document
a notification should appear in the stream indicating this applied to future activity
However, if the user is still seeing future activity for content they have hidden, I would suggest submitting a case in your customer group as a next step, so that a member of our support team can perform additional investigations specific to your instance.