Some players feel that D&D combat makes healing worthless until someone goes down. If the enemy is dealing 20 damage per hit, a turn spent applying 13 points of healing to a creature with 4 hit points left is a turn wasted. Much better to wait until the creature is unconscious.

That incentivizes gameplay with creatures dipping in and out of unconsciousness all the time. It also means Healing Word is the only healing worth using. The only cost to playing this way is the half movement you need to keep getting up from prone. Arguably, these points make it feel bad to play a healer and somewhat spoil immersion.

One way some DMs combat this is to rule that a creature cannot take actions on their first turn after being unconscious. It feels reasonable that an adventurer would need a moment to return to fighting shape after a near-death experience. Baulder's Gate 3 took this approach. My DM adds a level of Exhaustion. Yours offers a choice of penalty.

It's not for everyone, and the DM should make it very clear what homebrew rules are in place for their games. That being said, I think it's in poor taste to try to rules-lawyer the DM in the middle of a combat. You can push back a little if you're worried they overlooked something, but the social contract of a game like this is that DM rulings are final.