Mercs don't cast their curses on cursed enemies even if they have the curse limit.

Mercs won't cast their curses on enemies already cursed. Mercs that apply marks are completely useless if there are any curses already on the enemy. Flamequiver runs alch mark + blasphemy flammability so they will never even use their alch mark.

I have a 5 curse limit and without any other curses the merc still won't cast their mark because all enemies around them are already cursed. Curse on hit works because it's not using merc's AI to apply the curses.

Flamequiver is the only merc with a mark + a curse. They have blasphemy flammability so by default they will never cast their alchemist mark. I've tested with a multiple curse limit on them and dropping all other curse on hit so they only have their 2 default curses and they do not mark.

This makes flamequiver's kit bugged since release.
This hurts the flamequiver

Can confirm. My merc has a curse limit of 4, and triggers enfeeble, temp chains, and frostbite on hit. She never finishes the job and casts her elemental weakness.