Tappet adjustment failed and head assembly needs to replace

My 2023 hness engine made some knocking noise and I took it to ASC they adjusted the tappet but the noise didn't stopped and they told me it's guide noise so the head assembly or guide might be need to replace. I said okay as it is a manufacturing defect and it'll cover in warranty but my doubt is why does this happens?

Also the parts takes time to arrive atleast 2 weeks. They said I can use bike during this period and it doesn't affects any other parts.