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 like the surprise, the diversity in the fight, and to me the multi-phase bosses have always been extraordinary bosses. They would not have been so memorable if they weren't multi-phase, and pushed me as far as they did
(I started the Souls games with DS3, I have played DS3 & ER fully with DLCs, and stopped DS1 by Anor Londo)
The only thing I don't like about it is when I giga stomp a boss and am like? Oh? Oh we're done? Allright.