They can do that. And one of the first dudes you’ll see doing it is Margit. They only delay the time between back swing and the actual attack. So you’ll know it’s coming, you just need to dodge (or parry) when they take a swing at you, not when they bring their hand back. Shit, sometimes Margit will just do the back swing and if you stay away or calm long enough, he just goes back to an idle animation and doesn’t attack you.
The first measuring stick dude, he makes sure you learn the mechanics. I feel like Margit and morgott are the exclusively to make sure you have a handle on them. Hell I didn’t realize jumping could dodge attacks until godfrey.
The weird thing I found in the dlc with jumping is that, for whatever reason, when the Putrescent Knight does that blue fire attack, you can jump it but not roll it. Or at least, I can’t figure out how to avoid damage by rolling it. Rolling forward should avoid a hit, but I still take one; jumping it avoids it totally. It doesn’t even seem jumpable because the flames still touch your upper body while doing it and you only have i-frames from the waist down.
The fire is a persistent hitbox that lasts longer than a roll gives iframes. The airtime of the jump lasts long enough when timed right.
Some of the channels of blue fire can also be avoided outright but I never got a handle on how that worked. And the visuals, like a lot of fires in Elden Ring, don’t look like you should be able to jump it. Looks super edgy though when you do though.