In the spawn chunks, all entitys should always be moving. But if no player is in the overworld (all in nether/end), Falling sand entitys in the spawn chunks stop moving after a few seconds. This is a HUGE problem to mapmakers because falling redstone blocks are often used as clock to power command blocks which need to be in the spawn chunks to be always active.
Due to this bug, a lot of command creations that use falling sand as slow clock stop working if all players are in other dimensions that the overworld.