DK Experience Bug

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.

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

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

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