When a command block contains a setblock or blockdata command which creates a JSON sign with scoreboard data, the sign is blank. This appears to affect versions 1.9.3 and later, with 16w15b and earlier version executing the command correctly. I have looked over change logs and saw no mention of such a change around 1.9.3, so I apologise if this is intended behaviour.

Steps to replicate:

• /scoreboard objectives add cm stat.walkOneCm
• walk around a bit to populate the scoreboard score
• place command block
• place sign directly on top of command block
• set command block to
  • /blockdata ~ ~1 ~ {Text1:"[\"\",{\"score\":{\"name\":\"PlayerName\",\"objective\":\"cm\"}}]"}
  • (replacing PlayerName with your player name or @p)
• activate the command block