I have just recently killed the Wither in my solo legitimate Survival world on my PS4, Bedrock Edition. I got "The Beginning?" trophy once I spawned it in, but then once I killed it I didn't get the trophy for doing so, which is "The Beginning" (no question mark). I never enabled cheats or Creative during the fight. All I did was when I finished half of its health, I logged out of the world, made a backup copy (which I never used) and turned down the difficulty from Hard to Easy. This should never affect the world's legitimacy in any sense, but it might have contributed to the bug somehow. I even earned the "Beaconator" trophy and the game doesn't realize that there is no logical way I could have earned it without having killed the Wither. During the second half of the fight the Wither was also causing massive amounts of lag, since it was spawned inside a tight self-made cave causing its dash attack to destroy massive amounts of blocks, so that might have potentially caused the game to be irresponsive when the Wither died. I don't know whether or not that bug is specific to PS4, perhaps because of using the PS4 Trophy system as opposed to Xbox Live achievements.

Steps to Reproduce:

1. Spawn the Wither in a legitimate PS4 Survival world that is open for trophies.
2. Log out during the fight and change the difficulty from Hard to Easy.
3. Finally kill the Wither all the way through without ever enabling cheats or Creative.

Note: Spawning it in a tight cave might cause massive amounts of lag, so preferably also try doing so since it might be a/the cause of the problem. Also feel free to test this issue on other platforms to makes sure whether it's PS4 exclusive or not.

Expected Result:

"The Beginning" trophy/achievement should be earned once the Nether Star drops.

Observed Result:

"The Beginning" trophy is not earned despite the world being open to trophies/achievements.