Of course it's going to be fixed, but these are snapshots and large bugs are often pushed until all the features are in.
Running "/debug report" on a few seeds under both 1.16.5 and 21w07a, I am not seeing any dungeons (spawner with chest nearby on same Y level) near spawn on 21w07a, even when the 1.16.5 world shows several of them. Several spawners on 21w07a seem to be at the exact same X/Z locations, but 11 blocks higher on Y.
This is NOT a duplicate. MCL-13519 is about the launcher saying the game is already running when it is not. This is about that message being confusing when the game is, in fact, already running.