Certain block entities, when newly placed next to a power source, won't activate until they receive a block update.

Steps To Reproduce:
1. Place a command block to where it will be powered as soon as it is placed.
2. Type a command into the command block, e.g.: "say test".
> Notice the command block won't output "test" although it's powered.
3. Cause a block update next to the command block.
> Notice the command block now outputs "test".

This also occurs with dispensers, droppers, and note blocks.

See also:
MC-51340, it also happens if you got certain block entities with NBT data (e.g. a command block with an inserted command) and place that manually next to a power source, as well as
MC-31100 if you setblock certain block entities with NBT data where they would get powered, as well as
MC-80901 as falling block.

Code analysis by Marcono1234 in this comment.