I contacted support after seeing this terrible update and wondered if the new built in browser was Edge based (it is). Microsoft seems committed to forcing their new browser on everyone including Android users :/
This is the response I received from the Outlook app support team:
This is an expected behavior in the app. Link in an email or event in Outlook will now open in an integrated Edge webview within Outlook. This helps keep our users in the context of their email while tackling quick tasks.
If you wish to go to your default browser, please tap the overflow menu at the bottom and choose Open in Browser.