That's not a bug.
It's just like when you change a certificate, that adds +20% Mcrit DMG, you will gain +0.20x mcrit damage.
So, when the Behelder debuff is activated, you will gain more 0.50x mcrit DMG, and so.. when you hit 10k critical with 3.00x mcrit dmg, with behelder debuff you will hit like 11,6k.
If you crit. 10k, your base damage is like 3333. So, just make 3333 x 3,50 and get your final damage: 11,6k +/-.
Hope you get it ...