Is that supposed to affect only one single die, or any/all of the dice? This isn't the first time I've found GW's use of the singular frustrating - it come up in one of the new Vigilus Ablaze chaos legion's rules as well.
Rules as Intended I think it's clear it's every roll. Rules as worded is unclear. I would err on the side of it NOT being a single hit roll but all the hit rolls in an attack. Luckily GW is good with FAQs and rules clarifying in 8th edition.