When the ref stops the fight is irrelevant. The time on the tv screen is irrelevant. What matters is what the official timekeeper decides is the time and what the ring announcer declares is the official result. The official result on the WBA's official results page is the fight was stopped AFTER 1:31 of Round 6. Officially there was 1:29 remaining in Round 6. For betting purposes the fight went OVER 5.5 Rounds. Wikipedia has the same result. I haven't checked the WBC or IBF page. The YouTube links i've looked at don't include the official result being read by the announcer. The only way I see this being overturned is if someone posts the full fight video with the official ring announcement saying something else.
wbaboxing.com/boxing-news/the-bell-rang-in-las-vegas-stevenson-knocked-out-behind-closed-doors
wbaboxing.com/boxing-news/the-bell-rang-in-las-vegas-stevenson-knocked-out-behind-closed-doors