Guild Wars 2

Successfully landing Head Butt on a target and then immediately breaking the self stun portion of the Head Butt with Berserk Mode does not currently proc Eternal Champion. You do not get the x3 stacks of might nor the extra tick of stability.

Please fix.

I’m not sure how you tested this, but I tried it with with Savage Instinct selected in my traits, and it did trigger Eternal Champion just fine. The trait also works if you break the self-stun with abilities like Outrage or Endure Pain.
Keep in mind that going into Berserk is not a stun break unless you trait for it.

There is however an actual bug with the self-stun from Head Butt breaking trait and rune triggers (including Last Stand, which should work regardless of being stunned), that I have reported and tested here:
Head Butt self-stun breaking rune and trait triggers