Player is ghost (invisible / unable to "generate" world by walking)
The server thinks that I'm in a particular location - all other players on the server see the player there. When the player uses "/unstuck", they are teleported to that location (the location where everyone sees the player standing, stationary). However, the player is able to move around the world, invisible to all others. The player does NOT spawn new world blocks when approaching them, and will "fall off the edge of the world" if they reach the edge of the spawned world. They are otherwise able to interact with blocks - mining, logging, crafting, etc.
/unstuck doesn't fix it
restarting the server doesn't fix it
I really don't want to lose the world's progress....
Good news! It turns out that kicking the player from the server and then having them rejoin fixes the bug.