Cheap Shot isn't a consistent rune on Gangplank nor combined with Glacial Augment

Cheap Shot is a rune that applies bonus true damage to targets if the target is affected by the majority, if not all sorts of CC:

Wiki: leagueoflegends.fandom.com/wiki/Cheap_Shot

For some reason, it applies randomly when combined with the Glacial Augment rune (50% chance to work), and also will not ever work on Phantom Barrels or secondary barrels of the barrel chain, but will always proc on a single barrel with either Q or an auto-attack.