Infinite heal, death saves, biomant and RAI
We had a game session last night where we fought a monster that have stat block of biomancer. Its signature ability is a Bolstering presense: each ally within 30 feet regains a small amount of HP for free at the start of their turn.
There were also around a dozen lesser spellcasters (CR 1–3) with it. The Biomancer was healing them, while they were preventing us from getting close to it. There were fewer of them at the start, but because we were making a lot of noise, more of them came running in from nearby rooms.
The problem was that whenever we dropped these minions to 0 HP, they started making death saving throws. Our DM said that normally monsters don't make death saves because there is no one around to heal them, but in this case they had someone who could heal them.
We actually brought many of these monsters to 0 HP — Fireball was doing work — but they just kept getting back up, over and over, thanks to the Biomancer and the healers among their ranks.
We simply didn't have enough actions to get any of them to three failed death saves before they were healed again. In the end, we had to retreat after dealing a huge amount of damage, but without actually killing any of them.
So I'm trying to understand: is this really how the Biomancer is supposed to work? Is this RAI?
It feels extremely strong to me, especially when combined with a large number of weaker minions. Or is this interaction already accounted for in its CR? After all, it's a CR 10 monster, and my experience on that tier of play is pretty limited.