This happened to me too.

The day before the boss fight, I chopped down all the trees surrounding the boss spawner. Next day I get all my arrows and buffs ready and go to the fight. First half was normal, dodge ice sprays, go in with melee when it lands, killed a couple drakes. When boss got to like 33% HP, she landed on the edge of a slope and just sat there while I continually sent arrows to her face. She'd circle around a few times and then repeat. Very strange.

Honestly Bonemass with HP regen now is a harder fight. I first fought him back in April and was able to cheese the fight by making a treehouse next to the boss spawner and just use like 400 ice arrows. Now playing a new game tried this strat again and took him down to half HP before I realized I was doing negative damage with arrows. Had to drop down and smash him to death with an iron mace.