I don't think that's it. A little later in the game there's a different Novice Olenka replayable event that DOES count towards that quest.
Looking at it now, there's two Encounters with Olenka: "Fornication" and "Fornication2". BOTH end with spilling seed inside but only the second one could be used to satisfy that quest. Before it shows up there's just the one and it puzzlingly doesn't work.
The "replaying encounters doesn't count towards quests" instruction is a bit confusing in that "Encounters" and "Send for a Nun" and "Get Sophia" ALL DO count towards that quest (if you pick a proper sub-option for their completion), but they all only count once per day - the Quest counter resets to zero each night until you successfully finish it.
I have tested everything now, and I find no issues. I would need a save to show me otherwise.
I played the first encounter with Olenka and it did count toward "planting the seed" quest. But replaying won't, as it shouldn't do. Same thing with the second fornication encounter.
Note that when I say replay, and what counts as replay in the game, is when you have completed the encounter, having it end up in the "Encounters" button, and then playing it from there. The button for the event will be marked with an R in the top left corner. However, failing to fornicate with Olenka the first time(s) you meet her, doesn't mean it will be a replay when you eventually manage to fornicate with her in that encounter. Perhaps this is the cause of the confusion?
Since the quest is to creampie 5 times in a day, the creampie counter will reset every day, yes.
"Send for a nun" will count toward quests when replaying, as it says, but replaying encounters will not, as it says. I don't see a problem here.
Edit: Another source of confusion could be that Olenka also has events in "Send for a nun" and one of them is called "Fornicating".