This meant that I only had about 4 minutes to kill the boss and I'll come back to that later.

Right, and 4 minutes should be plenty. My healer is also Noelle, and she could beat down any Bounty boss solo in under 4 minutes even if the boss was immune to Geo.

My main team is Anemo MC, Noelle, Klee and Mona and the last 3 Liyue Stonehide Lawachurls I have faced have all been immune to pyro.

This is fully your own fault though. The game tells you what the enemy is immune to before you accept the quest, not all 6 options were immune to pyro. If you want to use OP DPS every single fight, only accept the bounties your OP DPS can do.

People were complaining about lack of infinite content. This is new infinite content. You're not forced to play it,

and you don't even get anything for it, besides 100 intangible reputation EXP.

You'd at least think that the bounty mini-bosses would have an increased drop chance for blue or purple rarity loot, instead they drop nothing, which is a joke.

They drop bounty XP and 30K mora each for 5 star fights. Don't whine about 150K resin-free mora a week, you can fight non-bounty bosses 24/7 without resin if you need more mats.

Lastly, which is the second part of the title, why don't "quest-related" mobs drop any loot?

Because people complain about lack of resin-free content. The current system means that "mat farming" is it's own unique activity, if quests dropped that loot you'd never have a reason to explore and fight random guys for their loot.