I was playing Khazan yesterday and fought Viper who gave me a lot of trouble for some reason. After struggling through its health bar and thinking it's finally over, a cutscene plays and a hidden second phase starts. At first, it kinda pissed me off, but after giving it a few more tries, it was fine in the end. Still a little annoying.
Other soulslike mainstays do this quite often too: Godfrey and Hora Loux, Malenia, many of the Lies of P bosses like King of Puppets, the Green Swamp Monster, Nameless Puppet.
Sekiro is quite upfront with its phases since the red dot indicators show how many healthbars a boss has. Apart from Guardian Ape and the final boss (if you know you know), I'm not sure if there's any bosses with surprise second phases.
Sister Friede does this in DS3 from what I've heard? Haven't played the game yet
Do you mind when bosses do this? What's your reaction? Does it make you want to fight the boss any less?
I always take my first try as a test run. I have managed to beat bosses on the first try because I still put in the effort to improvise and adapt to them, but I plan around losing. As such, it's not been that annoying to me in general, pretty much serves as a nice adrenaline shot. If we're talking about Malenia, though, the situation changes!
I hate when a very, very tough boss has a hidden phase. Attrition is nice, but not when I don't even understand if I can dodge them, let alone not when the boss forces me to burn through my utilities during the first phase. Something with a gimmick to it, like Mohg, works way better, I feel. Malenia's just a meat grinder *which dumps the remaining chunks into a wood chipper.