https://wccftech.com/343-industries-warns-halo-infinite-campaign-quick-resume/

Halo Infinite developer 343 Industries has warned campaign players to not use the Xbox Series X|S Quick Resume feature as this will prevent earned cosmetics to show up in multiplayer.

Apparently, using the Quick Resume feature can result in becoming disconnected from services, and, in turn, this could prevent users from using campaign-earned Spartan cosmetics in the multiplayer inventory. On Twitter, Halo community director, Brian Jarrard, explained what’s going on, and promised that the team is working on a fix.