Perfect Inscriptions - Trait functionality/bug?

Now that we are stacking Dragonhunters a lot more in Hall of Chains I've noticed some inconsistencies with how the active effect of Perfect Inscriptions functions. At first glance, it appears to give a stacking buff (which is why we use it with Bane Signet), but I've often noticed it end abruptly when it hits 4 stacks, rather than ticking back down like other boons would. It also appears inconsistent in being applied; we tested with all the Dragonhunters in a party standing next to each other, but it seemed like no two Dragonhunters got the same number of stacks when we all cast Bane Signet together.

Just wondered if anyone else had noticed anything like this, or knew why it was happening.