Sorry about the prolonged battle there. Unfortunately, this very likely isn't a bug.

However, the Spirit Keep boss heals itself by 50% of its max HP every turn, which means this likely isn't a bug. You need to be able to use champions who can affect the turn meter, apply heal reduction, or later on in the game you'll be able to just find a combination of both. Alternatively you can beat the boss with fast champs and damage, but that can be very tough early game.