Hm, the only thing I can think immediately is either soil or pH. Miracle fruit plants really like soil to be acidic, but they are also really sensitive to fertilizers and all that. Generally the recommended soil is a 50/50 mix of peat and perlite, with up to 20% coffee grounds mixed in for acidity, and a weak azalea fertilizer once its established.

My other thought is sunburn. I had a lemon tree nearly die because it got too much light without being hardened off, and it did result in similar burned spots on the leaves. what kind of light does it get? If it's in a window of sun, for example, and you have water (or other, like insecticide) drops on the leaves they can act as a magnifying glass and burn the leaves where the water was sitting.

They are renowned for being easily chlorotic. I had one that was in a greenhouse. So it had plenty of water..but the potted plant needed constant doses of iron.

I wasnt given enough space to type the full novel I wanted regarding
this plant, but I water with acidified water, and it gets a foliar
macronturient spray every now and again, you can see the brown residue
on some of the leaves, and this has supplemental iron.

My
other two miracle fruit plants are doing great, on the same watering
and nurtritional routine. The only difference with this plant is it is
in a different container mix, and it is in a self-watering container...

Then they are either drowning in water or, you rely too much on the pot and its going too dry between waterings. They look anemic. Better to try another pot and better mix.

yes, if you have two plants and one is doing poorly while the other is doing well - Barring pests on one plant, I would say whatever is different from the one that is doing well is the issue. In this case, if the other plant is doing well in a different mix with a different kind of pot, put the sick plant in that mix and in that kind of pot.

Is that a lab? Doing research on Miracle plants?

I had a lot of problems with my miracle berry plant with tip burn due to using chlorinated tap water. Once I started setting out a bucket of water for a few days so the chlorine would outgas, I had less of a problem. This winter I stuck an unused trash barrel under the eaves of my roof to collect rainwater. I only water the miracle berry and a blueberry bush with this water and it's lasted until now.