DK Experience Bug

[Lionmind]

Hello,
I am playing on Onyxia (WotLK 3.3.5) and I am experiencing a persistent XP rate issue that is NOT related to the XP bar, UI, addons or commands.
Situation:
Two players, two different accounts
Same party, same mobs, same zone
Zone: Borean Tundra
My character receives x1 XP
Party member receives x3 XP
Yesterday it was working fine, earlier (Friday) it was not
Server uptime is 2 days, so this did NOT happen after a restart
What I already tried (please read before suggesting basics):
No addons at all
Right-click XP bar → rate already set correctly
Logout / login multiple times
Party disband / reinvite
Changing zones
Hearthstone / travel
Same quests, same mobs, tested side by side
Commands like .xp / .rate are not available on this realm
This clearly looks like a server-side XP flag / sync issue, because:
It affects only one account
It changes without restart
It happens specifically in Borean Tundra
UI shows correct rate, but actual XP gained is x1
I am not asking how to change XP rate.
I am reporting a bug where XP rate desyncs between accounts in the same party.
Has anyone else experienced this on Onyxia?
Any confirmation from staff/devs would be appreciated.
Thank you.

[Shred]

This only happens whilst you are in party? ie do you get the correct experience if you are alone.

[Nobuemon]

What are the levels of you, your party member and the mob killed. XP gained is a formula

[Lionmind]

We are duo, same level, couple playing next to each other and we check the xp. It is higher to her x2. Also, we are in a party, but happens only in Northrend