I have tested this out 100 times and out of those 100 times 27 times I entered the game and suffered fall damage resulting in broken bones or death while logging in in the middle of a field.
When logging in inside structures it happens more frequently, 32 times out of 100.
1, log out in a field / building
2, log in and chance of suffering fall damage
rinse and repeat.
Recently happened to me as well, one the 2nd floor of a barn. Very annoying, might have something to do with slower connections; loading the player location before it loads the environment, causing fall damage due to player falling through nothing.