Yes, screamer cycles do go pretty bad sometimes (well almost every time to be honest

)
But as far as I know there were no changes on how "heat generation" works in A21 compared to A20.
So this could have happened to you in A20 in the exact same way more or less.
What might be different is:
Was it an infestation quest? Having 25 zombies spawned in and being after you at the same time seems a lot,
and maybe it wasn't really the normal amount for a "dev intended way" to clear a poi.
Did you run through a whole area and set off several spawn points at the same time? I mean of course if you go bright and loud you draw lots of unwanted attention.
And in addition to this, with the new magazine system, you might not have been that well eqipped considering your weaponry as you are used to from former alphas.
Which means you have to fire more rounds per kill, meaning generating even more heat.
One has to be careful in A21 not to fall for the fact: you still reach higher tier quests pretty fast, just by doing a certain number of quests, like in a20,
but your progression in armament/equipment is dampened by the new system. So you are not as prepared for the higher tier stuff as you might remember and expect from A20.
Well that's part of how they increased difficulty.
So what I mean in short: Ask yourself if you really were well prepared for what you started.
And then again, screamer cycles suck big time.