if you were using silverfish theres a line in its settings that says you dont attack with a weapon unless your opponent has 15 or less hp. if you were using deckrules then idk but maybe theres a similar rule in there as well. This isnt necessarily a bug pertaining to Jaraxxus himself.

its in the file HearthRanger\ExternBot\silverfish\settings.txt

the line is: enfacehp = 15; // hp of enemy when your hero is allowed to attack the enemy face with his weapon

just change the 15 to whatever you want if you always want him to go face if possible just set it to 30