Nah this isn't some glitch or whatnot. Mind you he was dying multiple times and this wasn't only happening with her but other bosses like this one big guy in a mask and a axe. He was also fighting some dudes in a coliseum ring with gold armor and pink mohawks or something. It's the part where the main character is wearing all black. It's hard to explain I need gifs but basically when you're in an attack, the animations kind of "reset" or seems to skip frames. It's mainly when she does this sort of spinning attack and I noticed he'd try to counter with his shield and often you'd hear the sword clash against the shield noticeably earlier than when it actually does which sort of threw him off.
Compared to other games Idk, it's similar to assasins creed now that you mention it except I'll give this game the edge due to the executions looking smooth due to it slowing down.
He was probably hitting the wrong button. With the bosses there is a specific patern you must learn. If you try to sneak an attack in when you are supposed to be blocking or shield bashing you will get hit.
With her spinning attack you need to block three times and your timing has to be perfect for the last one or you will get hit. It's best to just roll out of the way and attack her after her animation is over