Boss/enemy healing isn’t bad game design. There are a lot of very valid reasons that lead to more interesting interactions.

• A boss with slow health regen might require players to try to burst the boss down through weakpoints or debuffs

• A boss entering a second phase and getting their health back creates a cool “oh shit” moment and really emphasizes their power

• A boss that has an interuptable or preventable heal adds an interesting challenge and requires players to do more than just DPSing them down