Well, one major problem is it's hard to tell if she's being serious or not, but there are several times in A Flower Blooming Violet and IaMP where Yukari's basically like "You know this already" and Yuyuko basically replies "Know what?"
Although again, it's hard to tell if Yuyuko's being serious or just trolling Yukari whenever she does it.
My own theory is that Yuyuko's incredibly perceptive but only when she actually cares about something. Which, given her fickle personality and low (and sometimes seemingly random) attention span (she says she stopped caring about that body sealing the tree "just because." in one/some of PCB's endings, if I recall correctly) means that she can be really hit-or-miss on a lot of things. Yukari knows she's incredibly perceptive and thus assumes Yuyuko already knows lots of things, but Yuyuko's fickle personality and low (and random) attention span dooms even Yukari's assumption to be hit-or-miss, as well. Well, that's how I see it.
Though again, with the tree being public knowledge, she probably knows about it. I wouldn't be surprised if she randomly forgot and re-remembered it from time to time, though, given her personality. Though truthfully this memory/attention-span problem seems to apply to several old youkai. Remilia and to some extent, Yukari and Rinnosuke, seem to exhibit it as well. It's hard to tell if Remilia's trolling too sometimes, but we get to see things from Rinnosuke and Yukari's perspectives and honestly, their memories are kinda wonky sometimes.
....now I wish we got to see a narrative written from Yuyuko's point of view.
