I would also want to know how to change it back, ive been using opera since way before chromium change and has always as far as i know opened the same way, but there is an extension getting it 80-90% of how it behaved before slightly buggier, but i'd prefer it natively to be able to choose if i'd like this weird backward way or what ive always used.
extension is called Classic Tabs.
1.When opening a tab
Open all tabs next to the active tab,
and place consecutive new tabs in order
2.Open all tabs at the end of the tab bar.
however you prefer it.
and can change behavior when you close tab with extension.(if you want).