Rolling a crit on a longbow which has 3d8+2 for damage, the system rolled the dice, added the dice and doubled that total, then added the +2 at the end.
PF2e will add all relevant modifiers before doubling, so the +2 should be doubled as well. Notable exceptions being deadly dice and fatal dice.
With that said. The bow has a +2 to damage because of potency runes; however, potency runes only give bonus to attack not damage.
I want to clarify here what exactly is happening, in order to make sure I’m not misunderstanding what you’re saying:
It sounds like the +2 damage from the potency rune is what you’re talking about in that critical hit calculation, since there shouldn’t be any additional damage from the rune. Could you share this character sheet with me so I can take a look at what’s happening behind the scenes?
I believe you need to update your sharing settings for this sheet. If you click on your character’s portrait, it will open your sheet settings sidebar. At the bottom of that sidebar, you can update your share settings to allow others to view the sheet. 
Thank you, I didn’t know that was a thing yet.
I have it open now.
It looks like that additional 2 damage is coming from your Weapon Specialization.
Aw beans. I thought when I moused over the breakdown that it told me it was from runes. I’ll check that later.
Ah, I see the confusion.
I read the Melee attack section instead of the damage section. The breakdown shows weapon potency correctly but I misread that as the breakdown for the damage because my brain got stuck on the +2.
Oh boy, this boot doesn’t taste great but I guess I’ll eat it all day until I figure out how to read properly.
