Achievements suddenly unlocked for person joining me hosting

PC I7 6700k 1080 etc etc

How Not sure, just joining me made 7 achievements pop up, most related to Himfjäll that he hadn’t even visited.And he really wanted to earn his achievements.
Achievements it happened to:
Tunnel Rat
Elementary
Other Survivors
We’re Fully Booked
Outside in the Distance
Secrets Locked Away
Protector of the Resistance
Unorthodox Skiing (!?!)

I hosted and he were invited to get a little refill of ammo and directions. So I guess both Host and Client behaves odd when joining someone that have 100% the game, (and probably 100% the map too=.
Players in game 2

Time it happened


2 of them didn’t fit as they were higher up.
If more info is needed I let him fill in the rest at a later time.

Addition-> The Unorthodox Skiing unlocks if someone else does it while you’re hosting.

About repro of this, I’m open for any (organized) tests hosting, to see if it unlocks by mistake for new/test accounts.
No, not as in giving away achievements, more to get it confirmed that it currently always unlocks those when I host.