General Community Resistant to Healer Damage?

[Granyala]

Another example is if you've ever been dead against a boss that has ample HP left only to watch your team clear it anyway despite no heals from you because you're Ko'd.

That's pretty normal for content you outgear to triviality.
Sadly, that is the case with dungeons even if they are brand new, because they tune them to very old gear.
If you run them with the minimal item level setting, you will notice that creatures hit a bit harder.