Dragon fights issue solved
Ok I found a solution, first before trying this, if you have mods disable them (including cosmetic ones like reshaders, anything that alters the game files), otherwise do a fresh install of the game (to get rid of them).
Don't do too much damage too quickly if you have an op build, you have stagger the the dragon at least once every phase (I believe he perches twice per fight).
This worked for me.
The issue seems to be that you didn't stagger the boss before he perched, this was the problem for me at least, this was caused by my abilities doing too much health damage and not enough stagger damage which your melee attacks and other abilities do (so bring those abilities to the fight), and being a bit overlevelled since I'm at the point of no return and doing extra things rn.
