The Bug:

If you have the pervious output for command blocks turned on and you have a command in the command block that will give a previous output and activate it, no previous output will be added in the command block.

Example:

https://youtu.be/9Zl215WqHDg

Give yourself a command block and enter the following command:

kill @e[type=minecraft:enderman]

Run it and see no output

(On either mode X or O no command output is give for me atleast)