The advancement "How Did We Get Here?" is currently nearly impossible, because it at least requires you to get mining fatigue from an elder guardian, make your way into the end, and then get hit by a shulker, and also get hit by a Wither. Firstly, it might take a lot longer than five minutes to make your way to a shulker, especially if the player has unknowingly killed them already. On top of that, you also have to summon a Wither, who might just decide to target the Shulker for some reason.
Also, "The Next Generation" and, less so, "The Sky's The Limit" are basically impossible on a large multiplayer survival server, because both them ask you to find a very limited item.
These felt glaring enough to warrant a bug report. If I may suggest it somewhere else, please redirect me.
Not bugs.