https://www.pcgamer.com/steam-users-are-being-hunted-by-monster-hunter-world-notifications/

The PC port of Monster Hunter: World has a few quirks. Luckily, many of them have already been patched out and Capcom says its working on cleaning up the rest. The modder who fixed Nier: Automata's PC port is also on the case. But one of the game's strangest habits still remains: Steam won't stop telling players their friends are playing Monster Hunter. 

While you're playing Monster Hunter: World, Steam constantly sends out notifications to your friends which are usually reserved for when you start playing a game. We're talking once a minute, if not more frequently. Players have speculated that this is due to checks run by the game's multiplayer and that some wires got crossed in Steam, causing it to repeatedly confirm you're playing.