La cosa è stata prontamente risolta da Blizzard, che ha inoltre provveduto ad aumentare l'esperienza ottenuta dal completamento delle Spedizioni Classiche fino a quelle di Warlords of Draenor.
Ornyx ha scritto
From WoW’s earliest days, we’ve always had to take steps to limit experience gains for characters when grouped with someone of a drastically higher level. We recently became aware of a bug introduced in 7.3.5: a side-effect of the vastly expanded creature scaling was causing the logic to break when awarding XP in some situations. Earlier today, we applied a hotfix to creature experience that corrects this issue.
At the same time, we have significantly increased the experience awarded through queuing for random Normal dungeons from Classic through Draenor content. Running dungeons at the appropriate should feel rewarding, whether as a complement to or even an complete alternative to questing.
As always, we appreciate your feedback and will continue to keep a close eye on the pacing of the leveling experience in 7.3.5 and beyond.