They're looking to change it later:
From: https://www.reddit.com/r/thedivision...rch_11th_2020/To make things a little easier, the team plans to add an option to just reset the complete Manhunt, instead of having it connected to the Global Difficulty. This change will take some time, as the functionality to only specifically reset this is not in the game currently, and also needs more UI support.
So, until then how do players avoid resetting. If we keep our own global setting and run that way we are fine however if we join a group, other than manhunt, will that reset if they are higher/lower than my current setting?
Could the Devs sticky something to let the player base know how to avoid this reset. After all they should know how
since they play tested the expansion before release and would have been well aware of this issue.