it shouldn't need nutes at that age.
if it hasn't fallen over, it's not light-caused.
if you've been fertilizing too much, it could be lockout or toxicity related. pH is another thing to check.
So, your recent behavriour and environmental contexts will rule out the least likely.
if it is nutrient-related, it is immobile, since it's at the top. could be p / k tox or an Mn / Zn deficiency that all cause yellowing like that in that way. even iron is possible, since it;s a smaller plant.
So, rule out the easy stuff first -- pH / tds...
and stick to pH'd water-only. ~6.5