When making progress on Steam Achievements in game, only the host receives progress and all guests do not.

For example, when Reaching the hoist at the end of the first floor, only the Host receives 'Leap Before You Look'
This is the same for all achievements, including Reaching the Reactor for the first time, boss kills, or any other achievement.

This really de-incentivizes multiplayer play as achievements will not be progressed as a guest. It also forces groups of players to constantly swap who is host and lucky runs that should unlock harder achievements are wasted for guests.

Guests should be able to make progress on their achievements when in a multiplayer session.