I manage 8 listings and recently two of them have given me the problem you describe.
I send guests the check-in link instructions using the little key emblem in the app which Airbnb provide.
When I click on the link through my app, I am taken to a set of instructions. However a few guests recently say when they click on the link it just takes them to the Airbnb homepage.
i assumed they weren’t logged into to the app or the website, but they were. I saw the problem on my guest’s phone as I went to have a look myself. She had an Android phone, but I’ve also had guests with IPhones that have had the same problem.
Its clearly a bug that Airbnb need to fix, but when I contacted customer service they denied there was any problem.