Some truths are self-evident or prima facie, but many truths are not self-evident. It might be obvious in hindsight and that is frequently described as 20/20 (or perfect vision). Further, the truth might be said to be obscured, masked or hidden depending on context.
Finally, Merriam-Webster gives these antonyms to self-evident -
arguable, contestable, debatable, disputable, doubtable, dubious, moot, problematic (also problematical), questionable
The best phrase I can think of is still obvious in hind-sight. Such a truth is unarguable once understood, but clearly not self-evident.
Edit Inspired by this answer,
believe it or not.