[Snake]

That might be a critical hit with Fleur, which grants enough momentum to perform this:

Nothing -- (7M left)
Haunting (3M) -- (4M left)
Basic Hit [Critical!] (3M) -- (1M left, +2M due to Fleur)
Ghostwind (3M) -- (0M left)

This is 'how' you do it, whatever odd effects happening I don't know anything about it 'cuz I didn't test yet, but there you go. Also, this might be the only way to ever do this supposed 'bug'. Exploiting weaknesses for Momentum only grant +2M after two different skills hit weaknesses.

Might also be the wording. Ghostwind receives the bonus multiplier if you Haunting + Ghostwind normally, so the game's code would be expecting some sort of thing like:
--The last action done must be Haunting, so I can increase the damage in Ghostwind.
--If their last action is 'Basic Attack', then it does not make any sense to increase the damage in Ghostwind.
... Or something like that.