With my cross-wiki notification integration patch (https://gerrit.wikimedia.org/r/255328), if you have notifications on both wiki A and wiki B, and you open the notification panel on wiki A, you are shown an item saying you have notifications on wiki B, as you would expect. You don't see the wiki B notifications until you expand this item. However, when the panel first opens, API requests are sent to update the seentime on both wiki A and wiki B.
This is wrong, the seentime for wiki B should not be updated until the user actually views the notifications from wiki B.
(Probably also reproducible with Moriel's demo page in https://gerrit.wikimedia.org/r/254612 but I didn't try that)