I believe this is already reported for an earlier 18w version, but I wasn't able to locate the relevant post by searching and wanted to report that it still exists with 18w45a.  This issue was noticed, and tested by our staff, in a dedicated server environment.

When multiple players are logged into the game, and they are separated, the world seems to stop functioning.  Mobs will not spawn at night, plants will not grow, and leaf decay does not happen.  I'm not sure what other systems are affected by this, as it appeared mob spawners still worked.  Adjusting the gamerule for tick rate has no effect.

Some testing we did included setting the tick rate to 999 and observing both plant growth and tree leaf decay.  While both players were logged in, and separated by a good distance, the crops didn't grow and the leaves didn't decay.

At that time we had one of the two players log out.  Upon log out the leaves immediately vanished, and the crops immediately grew in the other player's location. 

We had the second player log back in and the objects again stopped growing/decaying.  We also noted at this time that no hostile mobs were spawning at night while both players were logged in, however spawning was normal when only one player was online.

We also brought the two players to the same location and noted that decay, growth, and mob spawning happened as normal.  The bug seems to only appears when multiple players are online, and those players are in different locations.

It should also be noted that the day/night cycle doesn't seem to be effected, and there is also a possibility that grass spread and mob spawners were not effected either.  Villagers seemed to still exist in villages, and there were cases of illagers also being seen.