The built-in leech tagging only marks lapsed reviews of learned items in the reviewing stage - specifically, after an item has passed your learning steps and entered into the long-term scheduler, each failed review of that learned item counts towards the leech threshold. If you fail that first review of the day, the card is no longer in the “learned” stage, it’s in the “relearning” stage, and that means nothing that happens as you pass it around re-learning steps counts towards anything either. If my leech threshold is 2 (to simplify), then an item that I fail today and get right ten more times today but then fail two months from now and again get right ten times that day becomes a leech right on that second fail. If I pass it today, fail it two months from now and then it keeps failing to pass through the relearning steps fifty times, even if it carries over to the next day or across weeks technically still in relearning steps (not that this should actually happen), it does not get tagged.
It would be extremely useful to be able to sort items easily by how well I “learn” them. And I could see something that applies tags according to triggers, for example, having triggers like “failing x number of learning steps in a day” even if it isn’t explicitly framed for this purpose overall.