I'm curious as to why you'd think that.Something that occurred to me while reading your post is that this is how 3e and PF1 were supposed to work. Two CRs are equal to a CR+2, and a CR+2 is supposed to be twice as difficult. Of course, that’s not how it worked in practice.
I played 3rd edition for a long time, and never got this impression.
It was only very rarely a problem for the characters that the monsters regrouped, since "regrouping" implies "there's so many of them they're not individually threatening" and if they're not individually threatening, a Fireball handles twice as many just fine.
In contrast, it's really only now at ~15th level I'm starting to see Pathfinder 2 work that way. The four heroes just fought off eight Level 12 monsters (gogiteths) like it was nothing. And they didn't even need to use any area effects. (They crit easily, and two or three crits easily deal 250 damage. Not to mention how they popped them left and right with Scare to Death)