I had a similar issue. I wanted a startup page for the Notes section of a channel I created. It was not the first page I created but it was the first page in order. The Notes tab was opening with the first page I created. None of my teammates had the issue, for them it always opened on the first page in order. I did however manage to make the first page in order my startup page.
The solution for me was to uninstall Teams and reinstall it. not really a proper solution but my guess is it is storing which is the default page in a cache somewhere and uninstalling Teams and reinstalling it seems to clear that default page from the cache.
It would be really handy if there was an option in the right click menu on a page to "Set as Default"