I don't use any anti tracking or ad blockers. I have the normal Reddit all and Twitter app on my pixel 3a and every time I click on a Twitter link it opens in an in app browser that can't log me in.

I think this is correct. My experience has been that when it's using a tracking url it fails (I Ad block and Tracker block) and I see the tracker url in the address bar, but when I click again, it just goes directly to the real URL; presumably some fallback Twitter put in to deal with people like myself :D