It's a bit of a hard thing to research as you can't infect people with a potentially deadly virus just to find out how to combat it. It's like neurology research. Most of our understanding of the brain comes from brain injury. Usually car accidents. But you can't damage someone's brain just to figure out how it works.
Advice could change again too. It's unlikely that the mask advice will change now as the research is pretty solid but other aspects could change based on new research. That's not a bad thing. That's how the scientific process works. It's a learning process.
The WHO is a bit pathetic though. They do a lot to help. For example, they are responsible fro eradicating small pox, and shutting down many Ebola outbreaks. But their pandering to China is pretty bad. Especially things like refusing to accept that Taiwan exists.