Deinitely not variegated leaves here, it seems to me that the plant is not intaking all the right nutrients it needs for one reason or another. Here are a few things I would look into:
Over fertilizing - can cause the plant to "lock up" and not uptake certain nutrients.
Under fertilizing - maybe missing something essential in your current nutrient supply
PH may be off - this one would take longer to explain, but can result in specific nutrients not being taken up. Which nutrients would depend on the PH itself. If it is high then xxx doesn't get uptaken, if the PH is too low, then yyyy doesn't get uptaken. (see chart below)
Mites/bugs/aphids - some of these may be too small to see with the naked eye; some live in the soil (commonly missed by most gardeners)
anywyas, here is the chart I mentioned: in this chart you can see that when the PH is above 7.5, Manganese is not easily utilized; inversely, if the PH is under 6.0, Phosphorus has a hard time being taken in by the plant.
This shows why a PH of 6.8 is recommended for soils in general. (Plant species may skew these values)
This may not solve your problem, but if you plan to troubleshoot all aspects, I would suggest considering the above possibilities
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