The missiles have what’s effectively a flak shotgun shell at the tip, when they’re pointed at an object and close by it shoots flak in a tight cone towards the front.

Flak spreads the damage better and does more kinetic damage than trying to ram a plane with a missile and hoping the concussion from a the resulting explosion damages something.

You're talking about a single "dash" on the frame before it goes all white. First question, if it were a laser, would be what exactly are you seeing there? A laser from the side is invisible, there'd need to be dust there, or the air would need to have turned into plasma. I don't think either makes that much sense. Second question/problem would be… it would have failed/be malfunctioning because —

— pretty much all AA munition works by exploding in close proximity to the target and showering it in shrapnel. So this might even have "helped" the missle/shell against malfunction in its fuse. And considering that this is designed to work like that, and it's likely not the greatest quality work on the Iranian side, it's also possible that the thing is already exploding and just ejected some piece of intentional shrapnel (or unintentionally itself) early, ahead of the actual detonation.

Or the Iranians edited that "dash" into that one frame, it's not exactly like it's a reputable source and it's in their interest to confuse things. Maybe they want the US to believe that the countermeasures are malfunctioning and helping their attacks, so they turn it off…