Villagers in two separate trading halls of the same world have switched professions. They were all at least level 3 and therefore locked into their original professions. At some point they switched professions (and trades) but maintained their trading level.

Before noticing this, the world was experiencing a couple of bugs after the most recent update: (1) my character could not walk or climb but only hover across the map, and (2) my trident with loyalty remained hovering in the air after coming back towards me.

The mobility issues were resolved after reloading the game, but the trident disappeared completely. It was shortly after this that I discovered that the villagers had switched professions.