I've followed the same watering/fertilizing routine for each of the plants in my dedicated Habanero raised bed with great results, fertilizing with a 2-3-1 fish emulsion once every other week.

The only other thing I've applied, outside of regular watering, is blood meal shortly after transplanting the seedlings to help with light green leaves which seemed to have really helped, leaves have been big and dark green ever since.

The affected plant also seems to be under-producing compared to the others. Any thoughts / suggestions?

Clearly different. I would only be guessing.
My two Hab are incredibly productive. Just picked 60+ the other day to give away. Didnt even strip them, had still more on. May want to be taking the odd one out before others get infected..
But hope someone else has info.

I don't see any disease nor pest at work.

Hypothesis #1. Bad genetics: it happens from time to time to have a single deformed/underproducing/stunted plant out of the same batch of seedlings. Even industrial grade F1 hybrids can have that issue. Really nothing that can be done about it.
Hypothesis #2. The rootball developed poorly. Again very little that can be done: if there's a single plant out of an entire row with poorly developed roots the cause is generally just bad luck.

I agree with El Pollo, could be either #1 or #2. It looks chlorotic, as if it isn't taking up nutrients properly like its bedmates. Hypothesis #3: I sometimes have problems with the plants at the corners of my raised beds not doing as well as the ones in the interior. I notice that the corner soil dries out faster and as the season progresses the soil in the corners gets hydrophobic, so the water runs off and down the sides and they fail to thrive. I don't know how you're watering with the plastic in place, but it's a thought. Try giving it a shot of nitrogen. Blood meal is good, but might be too slow-acting for this time of year. Maybe extra fish emulsion or more often.
Nice crop!