When fighting the asuran boss at the end of the Uncatagorized Fractal, he will reset instantaneously if you use stealth.
This has been an issue for a while, but I bring it up now because this recent patch adjusted some stealth mechanics and even fixed the Giganticus Lupicus instant reset from stealth. I believe this Fractal boss fight was overlooked and should be adjusted in the same way the Giganticus fight was done.