These sites have a link campaign. Obviously multiple links are counting.
In your case Professor, you may be confusing a cause with an effect. Your PR goes down, and suddenly a lot of PR4 links now become PR3 and no longer show as backlinks. Just because they don’t show doesn’t mean that they aren’t helping you though.
The lower number of backlinks did not lead to the lower PR. Lower PR leads to less backlinks showing.
I don’t know exactly what has changed for you Professor, but you still do show multiple backlinks from some domains. Are you talking about missing links from pages where you host both the linked to and linked from site? They might be ignoring/downgrading links that are closely IP related, but as the above group shows, multiple links are still counting and offering a big benefit.