I was able to borrow an iPad for a while (iOS 15.5) - not sure if this info helps, but it seems when logged into my lvl 52 account I am having 0 issues visiting level 200 islands. So those that end up not able to log into their island, then not able to visit the island if they log into a lower level island - are you guys using the same device to do this?
I wonder if unecessary data is being saved somewhere on the device. And there is no way to clear the data without factory resetting - seeing as even clearing cache/deleting data does not let you start a new island.