Totally random question: are you using group initiative or individual initiative when you run PF2?
Individual initiative for the characters, but I've changed to doing group initiative for monsters of the same type - since the VTT gets very confusing when doing it otherwise. After we had the second TPK doing individual monster initiative, it just didn't feel like it changed the outcome that significantly.
Not sure the question was that random... Paizo clearly states (somewhere) that monster group initiative can significantly increase the difficulty of an encounter.
That is, you obviously always run the risk of all monsters rolling well and all heroes rolling badly, meaning all monsters go first, and if they all chomp on the same character, that character is dead with a capital D.
Grouping monster initiative makes that happen significantly more often. It can still happen with individual initiative, just not nearly as often.
Nothing wrong with grouping initiative. Just that the GM needs to be aware.
Simplest solution - make the monsters of a large or dangerous group spread out and find their own individual hero to eat, even if that means some of them just move on their entire turn (bypassing the already ganged-upon first character to reach the others in the back).
When the monsters are meant to be smart or cruel, thus be likely to focus their fire, make the players know this, so they can take appropriate caution.
Or use individual initiative for them. (They're not a group of just three cannibal cultists. They're Joe the Cannibal, Bob Soup-Master and Sue the Flenser, and since they're three distinct NPCs they roll individual initiative!)