When a command block is switched to Repeat mode after it was previously on Chain, the command block does not fire like it should. I have only tested this with "Always Active".

To reproduce:

1. Place a command block and open its interface.
2. Set the command to "say test" (or anything with a constantly-visible effect), and set the mode to Chain, Unconditional, Always Active. Save the settings.
3. Open the command block interface again and change the mode to Repeat.

Expected result: Command block should begin repeatedly running the command, as it is now set to Repeat/Always Active.
Actual result: Command block continues doing nothing, despite being set to Repeat/Always Active.

Ctrl+Shift+middle-clicking the command block in Creative mode (pick block +NBT) and then placing a new copy of the block results in the command executing from the new block like it should, but only from the new block.