Same thing in Black Myth Wukong.
In Hogwarts Legacy, which will allow you to change settings in game without restarting, this issue can be bypassed by disabling frame generation / relfex, which will immediately cause the GPU to throttle back up to full power, then reenabling frame gen / reflex and the GPU will stay at full power and perform as expected.
However, somethimes after some gaming, at random unforseeable intervals, the GPU will still fall back to this 780 MHz limit and stay there until doing this disable / enable cycle again.