I’d say it’s definitely a common thing.
All of mine in flower have this.
I think if ya zoom in you can see mine, some are “worse” the others but all have it.
Im suprize how close they are you dont have any mold problems ?
Nah, plenty of air flow.
Exhaust fan up top pulls air under the door blowing up through them from the bottom and theres 2 fans blowing from the top down on them.
My big tent is even more stuffed.
Big fan hit from the top and a oscillating fan for under the canopy.
Things touch but theres nothing “mashing” against each other and you can see all the stuff I’ve removed, taking entire limbs sometimes to fit as much as possible but still leave just enough for air flow.
And of course maintaining a proper humidity helps also.
I hope i dont get any mold . I saw this weird looking bump on the leaf stem . Ill try take a pic if i see another 1
You saying you have only 1 fan 
1 fan could be enough, can I see a picture of your entire plant in the tent?
It may be a good idea to defoliate some by taking big fan leaves off.
I try to look down through my plants from the top and defoliate so I can see down through my canopy. Doesn’t have to be wide open straight to the floor, but just enough so the wind from the fan can get through to the middle.
I remove some fan leaves then step back and watch to see if the leaves on lower stuff sorta move with the wind if that makes sense.
Man you dont even want to see how close the light it . Im still learn and next grow im gonna try to train when there still young but yea ill show you
It doesnt look as good as you guys grow . Lol as you can tell . Still got alot to learn
That’s a very nice looking plant. You have that in excellent health and should get some fire results by the looks of it
I was wondering how you got your plant to have colas like that ? Tie it down ? Or cut top off ? Next grow i want try do silmair grow as you did
Some plants just naturally grow long colas but it helps to top by 3rd or 4th node and then try get it to spread out as it grows trying to keep all or as many braas you can to grow up the same height, then right before flower completely remove all the lower stuff and any branches that stayed short or just didn’t make it to the top.
Heres some stuff I removed from these just now.
Maybe this kinda give ya an idea.
