@Suty I have this same issue too.
I have Windy Premium on iPhone 15Pro, iPad 15.9 Pro, iPad 11 Pro and on my Mac. It seems like I can only log into Windy Premium on one or maybe two devices. As soon as log in on one device, get logged out on the other. It is starting to get really annoying and there is clearly a bug somewhere. All my devices are on the latest iOs (18.0.1) and my Mac is also on the latest version. Windy app is also up to date.