There's a long old issue with MC where if you jump on farmland it will turn to dirt, but since your y-coordinate is slightly below where the ground now is (dirt is taller than farmland) you get "stuck" in the dirt and have to either (super-slowly) move to the next block, OR break the dirt block underneath you to regain full control of your player.

Since the upgrade to 5.1, this is happening with our users when they teleport, probably 3/4 of all teleports cause them to get stuck in the floor. Players will have to break the block they're standing on or fly (for those who have flight, but most don't) to "escape" the bug. For those who teleport onto any stone variant and don't have a pickaxe handy, this can take a while, is it takes quite a while to break stone without a pickaxe, and it is difficult to even make the player look at the floor while this is happening.

I'd imagine the issue revolves around the teleport either storing a slightly off Y-value, or implementing a slightly-off Y-value.

Have exactly same problem, started with v 1.50 on our server

We have experienced this on the official ATM3 server as well and is still happening as of 5.2 -- I don't know what's causing it.

No longer seems to be an issue as of v5.7