Hi There,
From my experience shopify sometimes does that with development apps/shops, especially if the shop hasn’t been migrated to the new admin.shopify.com domain, then when it comes back to the redirect_uri it adds a force legacy domain url attribute which then opens up a new tab.
so I would suggest first checking the dev shop url. ist it a myshopify.com domain or is it admin.shopify.com domain. If its the myshopify.com domain I wouldnt’ worry about it too much. If its not I’m sorry but from the screencast I can’t see enough to help.
Cheers,
Gary