This could be me being a n00b, but here goes.
I want to give keepInventory to only certain players using the essentials plugin. If I apply the permissions to a user with "/lp user $username permission set essentials.keepinv true" it works fine. But it's a pain to manually apply this to all the users I want individually, so I created a group as follows:

/lp creategroup keepinv

/lp group keepinv permission set essentials.keepinv true

/lp user $username parent add keepinv

And then smited $user to death. User dropped their items as though they didn't have the permission, even though the same permission works when applied to them individually. Increasing the weight of the group didn't help either.

the commands should be /lp user $username parent add keepinv

the commands should be /lp user $username parent add keepinv

Apologies, that's what I meant to write. Edited.

Just to make sure I didn't do it wrong initially, I tried that exact command again and the issue still persists.

Hold on a second, I figured out what I was doing. When I tested this on myself and my alts I was setting user perms to false instead of unsetting them. There's my problem.