Please also add a further check/instruction to uncheck "Always keep my total playtime private even if users can see my game details." in Steam.
It's shown on the screenshot, but the text on the website below does not specifically mention that this is a requirement for Steam Quests to track.
https://anz.alienwarearena.com/steam/questsetup
I know it may seem obvious, but some users that have had privacy settings turned on may miss this.