The X-PRO35 is from a X-KEYS Keyboard.

I even did something simpler (targeting an alt in duel):

Hotkey 4 (X-PRO35 worked too)
Hold any Keystroke... Off
Execute a step when... pressed or released

Advanced - leave as default

Created Step 1:
Keystroke action -> 4 (my affliction bind) -> self
Do not advance to the next step -> Unchecked
...for at least -> Unchecked
default value
and only trigger once -> Unchecked

Created Step 2:
Keystroke action -> 0 (buff) -> self
Do not advance to the next step -> Checked
...for at least -> Checked
21s
and only trigger once -> Unchecked

And it worked. I spammed buff whenever it was ready and casted Affliction every 21s.

Which makes sense, leaving it free to proceed to next step (buff) and only lock on it and then going to step 1 again after the lock.

Now, why won't it work having two steps of Affliction both with 21s lock?