Unshakable (Firbolg): Trigger per Stress or per Source?

Unshakable (Firbolg Ancestry): "When you would mark a Stress, roll a d6. On a result of 6, don't mark it."

If an ability causes you to mark multiple Stress at once, like Corrosive Projectile (Sage Level 3), which lets you spend 2+ Stress, does Unshakable trigger once per Stress marked, or just once for the whole instance?

For example: spending 4 Stress on Corrosive Projectile, do you roll 4d6 (one per Stress), or a single d6 that could negate all 4, or a single d6 to negate just 1 Stress?

My reading of the wording ("when you would mark a Stress") suggests it triggers per Stress, meaning you'd roll one d6 for each. But I could see an argument for a single roll representing the whole triggering event, with a 6 potentially canceling all or only one of it.

Which interpretation are others using?