The bug

Using /setblock to place an "Always Active" command block (or any of the command block's variants) which already has a command in it will not cause the command to run immediately. If you change the command block so that it needs redstone and then back to the "Always Active" state, the command will run as expected.

What I expected to happen was:

The command block would immediately execute the command, as it had done in prior versions.

What actually happened was:

The command block did not immediately execute the command.

How to reproduce

Set a repeating command block (make sure cheats are enabled) with

/setblock ~ ~ ~1 repeating_command_block{auto:1,Command:"say Success!"} 

->  The command isn't run immediately