The crit damage, together with the crit% chance is not shown in your chat window when you get hit by a caster nuke in a duell. This may apply to all caster nukes but I could only test it in a duell where it definitely did not show up. Neither did the damage itself show up nor the %chance, basically the completely chatline for the crit is not shown which leads to the pet dying to damage which you as the pet owner can't even see in the chatlog.