The referee was looking directly at the collision and waved it away, only when VAR told him to review it did he have a change of mind, VAR is ruining the game.
As mentioned, those type of coming together, either deliberately or accidently happen at every freekick and corner. Now suddenly it is a clear (well once VAR google search told them) foul.
I reckon a review of multiple other goals given this season by VAR would show the same incidents being ignored (and probably worse like West Hams goal this weekend where a player can jump in front of the goalie whilst in an offside position and be adjudged to not interfere.)
I've watched this incident a few times on iPlayer now, and it's clear the goal was disallowed for offside against JJ, NOT for a foul on Cavani. The ref gave an indirect free-kick, not a direct one, so we know for certain it was offside, not a foul.
My personal view is that the ref saw the coming together in real-time and thought "nah, no foul, just a coming together", which is the correct decision. But, once he was shown that JJ started in an offside position on the monitor, that "coming together" becomes "interference" as JJ is affecting Cavani's ability to challenge for the header. The severity of the contact between JJ and Cavani is - unfortunately - irrelevant. Just getting in someone's eye line is sufficient to be called offside, as we've seen. Blocking a run, even if it's not enough for a 'foul' anywhere on the pitch, is enough to pass the threshold for 'interference'.
I think this is why, if you watch the ref go to the monitor, the first thing they show him is the side view of the pitch with JJ's starting position - NOT the contact itself. Because the fact he started in an offside position makes the contact an offside offence, even if it's not enough for a 'foul'.
It's shit, and I hate how long it took - but unfortunately, I think it was the correct decision. If you let that goal stand you open the floodgates to all sorts of blocking moves on attacking free-kicks in future. We tried to be clever, and push the boundaries of the offside laws, and this time it didn't pay off. We'll learn and move on.
Even without that, we had more than enough to beat them last night, and they got extremely lucky that VAR was even in play.