An oil gallery in the rear bank seems blocked (oil puddle there that does not go away).
The one on the passenger side (of 3).

Is this a problem?
Can this be fixed without removing the head?

Considering the puddle of oil does not drain, I assume no flush will fix that.

Panic not!

It is normal for the rear head to have a puddle of oil in it. Look at one end for the drain, it is a big hole, could not possibly get blocked, unless you have dropped some marbles onto the rear head! lol (?)

The front has the same drain hole as well, but because the engine is tilted in the engine bay, there is less residual oil in the front, ie less of a puddle.

Max, the whole drain gallery is full of oil, this is not just an accumulation of oil but it's a blockage.

To add to my first post, there is zero sludge in the engine. The best idea I have is that maybe at some stage the rocker cover gaskets were changed and someone overused RTV?

For anyone doing a search in the future...

I called MitsFix and they confirmed that there is actually no oil drain gallery there and the accumulation of oil is normal as even though it seems like an oil drain gallery there is nowhere for the oil to go.

there is nowhere for the oil to go.

Not quite right, is it?
Pressurized oil is used to lubricate the rockers, camshaft and run the hydraulic lifters. So oil is constantly flowing up there, it must go somewhere - can't just puddle there until there is no space and the rocker cover lifts from the pressure. lol
The puddles actually fill to a point, then overflow to the main oil drain. You are just looking in the wrong place, don't expect to find drain holes at the bottom of those puddles.

Google some images of 6G heads and see if you can figure it out.

Consider this:
Assuming the front and rear heads are identical, and you add 4 L of oil during an oil change, how does the oil get from the oil filler - which puts the oil on top of the head - to the sump?

Max, I meant there is nowhere for the oil to go in that particular puddle. I can 2 oil drain galleries (with no puddles of oil), I assumed that the one in question was the 3rd but was blocked.

I stuck a thin flexible tool down and it went a good 10cm or so and came back up drenched in oil, hence why I assumed it must be a blocked oil gallery.

Just the two as far as I know.

The heads are the same casting, rotated 180 degrees for the rear bank. The front bank, front lh drain hole lines up with a corresponding drain hole in the block.
The rear bank (rh rear corner) of the block doesn't have the drain port cast into it so the oil will pool in the drain hole in the head. We're talking maybe 50ml at most.
Even the head gaskets have a cutout for the non existing hole lol.