How to deal with this GM?

[DeTess]

Ah yes, pop up healing and NPCs ignoring downed players who are just going to stand back up. Always a classic 5e expectations issue. Glad to see you got this one worked out.

I get where you're coming from, but after a boss wipes out the frontline in 1-turn alpha-strike, trying to heal people so that running doesn't result in half the party dying does feel like it should be an option. Like, it wasn't a case where we where deliberately pop-up-healing. Rather, my fighter and the paladin went down, our druid started dropping healing spells and the boss immediately stabbed said characters again to make sure they didn't take any sort of action, which therefore included trying to run away.

It should be noted that the DM has also started running a houserule where death saves are rolled in secret by him, which encourages pop-up healing even more, as you don't know how close to death someone is.

edit: I think another part of the issues me and TinyMushroom are having is that this is a group in which we've both been playing for a long time, with differing DM's, and all previous campaigns (with the exception of Shadowrun, where cunning and brutality is part of the system expectation and better supported by the rules) had very much been a lot 'softer' and focused on heroics and story stakes rather than 'pick the wrong fight and you're looking at a potential TPK, or at least a very painful slog to get out of it'.