Alright, an update: having logged in the game with a previously completed event track, I’ve received just one ordinary daily quest.
Thus, according to everything said above, I must conclude that the quest system with the event is just bugged once its weeklies are done. To summarise, apparently, only one daily event quest is received after that point (until the whole event track is done), not two, and it is ‘hidden’, that is, not displayed in the quest log, but can still be progressed and completed (not re-rolled, though, as far as I can tell). However, two ordinary daily quests are received instead, until the event is done. The working theory at this juncture, I suppose, is that an extra daily quest (an ordinary one) erroneously replaces a second daily event quest, and the whole system with the log etc gets glithced at that point.