My humble opinion and advice - Nitrogen, phosphorus, potassium, magnesium, chlorine and zinc* are mobile elements that the plant will take from older leaves and relocate to newer growth if there is either a deficiency, lockout, or other uptake problem (since you stated it is only affecting lower growth). So one of those elements is likely to be your culprit.
The easiest thing to check as far as ruling out an uptake problem is checking that run-off is within a good pH range. If pH is within range, you'll have to differentiate between a true deficiency or a lockout/burn.
Most lockouts/burn are easy to identify and deal with through the simple step of reducing nutrient strength. So that is where I would start if pH was correct. The easiest way to do this since the problem appears to be minor, is to flush and then resume feedings with the strength of given nutrients down by half (in your case to quarter-strength since you were already giving half-strength) and see if the problem resolves (then it is/was a lockout from too strong of a nutrient mix and problem solved) or worsens...
If the problem worsens, you'll have to trouble shoot more and try to identify a specific deficiency or disease and supplement/treat it. But I would try the first two steps first before adding anything extra and possibly making a minor problem into a bigger one.
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*Zinc is variably mobile like Brick Top correctly pointed out below. Mobility of zinc within a plant is dependent on "degree of deficiency, plant species, and nitrogen status of the plant." Just for clarification/as a correction. Thanks.