Yup, I had this happen a couple of times back near launch. I remember reading about it being a bug at the time, but I'm not sure how widespread it was. If that's true, perhaps you lost it sometime last cherry blossom season? I never found a way to fix it, other than eventually finding the same diys again later!
So I restarted my island several months ago and sometimes I would notice I didn't have a DIY that I remembered learning or the things I ordered never came but I always assumed it was user error. Like maybe I closed the game before it fully saved (although I wait to see the dialog box saying I did).
This happened to me too! I got the cherry blossom petal pile and it let me learn it even though I know I already had it, because I traded my extras last year. And then the same happened with a piece of Easter furniture, though I don't remember which one.
At least I know I'm not losing my mind and this is something that actually happens. Now that you mention it, I do remember this being a thing last year, but since it got fixed I'd clearly forgotten about it.
I guess I hadn't tried to make the cherry blossom branches since this time last year, so hadn't noticed that it had vanished until now!
