After saving a newly created custom world and loading it immediately after, my avatar starts falling through the floor. The game is in survival mode (despite setting creative mode as defaultgamemode), cheating is enabled. But I cannot use "/gamemode creative", I cannot use "/tp @p". Finally my avatar is stuck on the world base level. I can "exit and save", and next time I load the world, my avatar appears on the ground level again, but starts falling though the ground immediately after appearing, the world seems to be still built while appearing.

The issue disappeared when I closed the game and restarted it. So it seems to be a rare case.