https://wccftech.com/blizzard-wow-classic-xp-bug/

Blizzard has rolled out a new hotfix that fixes a WoW Classic XP bug that scrambled an XP penalty that was in place in original World of Warcraft.

Following player reports, Blizzard identified a bug that affected how XP was calculated for parties of 6 or more players.  This bug was present on all realms and following this new patch, XP should now be should be significantly lower for parties of 6 or more players.