The only time you might be able to convince me that it's acceptable is if this account is only for one app and everything is all first party. If there's only ever one app then there's effectively no OAuth and everything (including the AS) is part of the app.
Frankly the "system browser is horrible UX" argument lost a long time ago once the OSs provided in-app browsers that share system cookies but aren't visible to the app.
I still use it every day and add new venues and everything too! It's still the best source of up to date venue information anywhere, usually even beating Google!
This particular issue isn't really a problem if you control the app and AS, but there are other reasons not to embed the AS page in an in-app web view.