Probably introduced because of boat/riding changes; minecarts (not sure about boats/pigs/horses) do not send nunloaded chunks to the client.

To reproduce build a long minecart track and ride on it. When you reach the end of the chunks that were loaded when you got in, more chunks will not load. The F3 screen displays "waiting for chunk...". Relogging or getting off loads the chunks again.

This is purely client-side; a friend on the server could see me riding normally in the minecart.