Are you able to share a clip of this with us so we can share with our team?
If so, could you post it in a ticket at #request-support over on our discord.gg/phobiesgame discord server?
I have a similar problem but other conclusions - I’ve noticed that whenever he gets attacked by some combined “arms” (like base damage + stim pad / electricity / fire / poison) he gets the bad end of the stick; base dmg + stim pad seems to do full damage and attacks with whichever element do base dmg with no burrowing taking -50% effect.