Hi there, you are through to Dr Linda, a UK based vet.
I'm so sorry to hear you're having this issue with Yuzu and appreciate it must be a big worry.
The most likely reason is that the kitten smelt different when she arrived back. This will be because of the smell of the hospital, any other animals that were there etc.
The older cat will also likely pick up on the fact that the kitten is acting a little differently and may be a little stressed.
Even in cats who get along well, this kind of event can set their relationship back a little.
Take a step back and go slowly, as you did when they would have first met.
Give each of them PLENTY of space.
I would also highly recommend a Feliway Friends plugin. You might want to consider some natural calming supplements for Yuzu too.
Hopefully they will be back to how they were in a few short weeks.
Any further questions please just let me know.