Chalk that up to reading comprehension failure - I didn’t stop to think it’d be weird to go from talking about the Abyss Watchers to the Soul of Cinder.
I have no trouble believing that the watchers’ second phase does more poise damage. I can’t remember how that fight went for me last time (NG++) but it probably wasn’t a big deal due to having Havel’s Greatshield. If you’re not shielding and only using the axe, I imagine spamming Falling Bolt would stop working sooner or later. Given that they stun easily you’d have more success with regular attacks.
Falling Bolt used to be almost impossible to interrupt, and that’s all changed after the patch. The most obvious is that most light armours have less poise now, and most heavy armours have more, meaning that if you want to use poise-heavy attacks like Falling Bolt you’re going to be much more easily stunned if you don’t wear heavy stuff.
The other interesting change is that poise has gotten a lot more conditions for recovering. Instead of getting your poise hit until it depletes, heavy weapon swings will now refill your poise to 80% if it wasn’t already higher, and weapon skills refill it to 100%. That means two things: If you swing your weapon and damage >81% of someone’s poise, they can poise through that with their first swing and refill it back to 80%. However, hitting them a second time will not allow them to complete their swing and stagger them.
If your weapon damages them for less than 80% of their poise, swinging their weapon allows them to refill their poise enough to never get staggered. So it’s simple. With any given weapon and any given poise value, you either stagger on the first swing, the second swing, or never, as long as you keep swinging.
I don’t think I fully have my head around it either but that’s what I read.