According to Spigot's JavaDocs, the PlayerInteractEvent runs as cancelled if a player interacts with air. Quote: "This event will fire as cancelled if the vanilla behavior is to do nothing (e.g interacting with air)."

This should not be the case, because right-clicking the air with an armor piece can equip the armor piece in your hand. A cancelled event suggests this is not happening.

To demonstrate the issue, I have included a plugin which broadcasts two messages - one from listening to the PlayerInteractEvent with ignoreCancelled = true, and one with ignoreCancelled = false.