There is some kind of desync happening between scoreboard displays and actual scores. Most likely because some boundaries between "no score" and "score of 0" have been destroyed.

The most obvious example is the new "scoreboard players get" command. This command, when used to retrieve a non-existing score, will initiate that score as 0 and return that. This by itself is not really a bug (although I do not agree with this behavior), but when the score gets initiated it does not properly update on scoreboard displays.

How to reproduce:

• Start a fresh world in 17w45a
• Make a new scoreboard objective

  /scoreboard objectives add foo dummy

• Set the sidebar display to this objective

  /scoreboard objectives setdisplay sidebar foo

• Get the score from the player

  /scoreboard players get @s foo

  This will return the score as 0, yet the display does not update.

• To verify that the score is actually initiated, use

  /say @s[scores={foo=0}]

  This will indeed find the player.

• The display can be manually updated by clearing the sidebar display

  /scoreboard objectives setdisplay sidebar

  and setting it to "foo" again

  /scoreboard objectives setdisplay sidebar foo

  The score of 0 is now indeed visible.

Be aware that this is just one example, I've had many weird desyncs happen (including a "scoreboard players get" initiated score sticking on the scoreboard when resetting the score). But this is the only one I can reliable recreate so far.