This behavior was changed in newer Opera versions. Alt + Enter opening a URL in a new tab is no longer supported in the address bar, especially in Opera Developer builds.

Current alternatives:

Use Ctrl + Enter to add .com (same tab)

Press Ctrl + T, then paste the URL and hit Enter

Middle-click (mouse wheel click) on the address bar after typing the URL

Right-click → Open in new tab

Use Ctrl + Shift + Enter (works in some builds)

At the moment, there is no built-in way to restore Alt + Enter behavior. The only workaround would be a keyboard remapping tool or extension, but Opera does not officially support this shortcut anymore.