Added 30 minutes later
I remembered. I encountered a bug. After taking the "healing" skill somewhere in the middle of the 3rd or 4th map (I don't remember exactly when), this healing skill stopped working (became inactive), even though there were no mana or other restrictions. Apparently, this happened after Irina started walking on water. I'll try resetting the skills after reaching the first level of Blood :-(. Steam license.
I reset the skills. For safety, I didn't take healing; instead, I took regeneration.