What are your "N" and "P" ferts? My immediate guess is a potassium deficiency as you aren't dosing any of that explicitly.
You may not have seen this deficiency before because there was some other deficiency that took priority over it. For example, if this is a potassium deficiency that's just now showing up, it implies that perhaps the plant is now demanding more potassium than it used to, either because it's grown or because an old deficiency has been satisfied (a phosphorus deficiency for example). Now that this old deficiency has been taken care of, the plant is now limited by this new deficiency.
There's a description of plant nutrients (called Liebig's law of the minimum) that the growth is limited by the lowest available nutrient. Think of it like slats (staves) of different heights in a barrel. Each slat represents a nutrient (light, CO2, nitrogen, phosphorus, potassium, micros, etc). Now you begin adding water to your barrel (analogous to the plant growing), eventually you hit a point where the water starts spilling out of the shortest slat (this represents your deficiency) and you can't fill the bucket until you replace the shortest slat. Now you fill some more until you hit the next slat and can fill no more.
Here's an image of what I'm talking about.
In reality things are more complicated, but the analogy works.
As you begin adding ferts, you'll hit different levels of deficiency as the plant grows that weren't previously observed. So, I'd say start dosing some more potassium, usually the new leaves look nice and the old leaves are "decaying" away.
Sorry for the long description, just got on a roll, haha. Hope it helps!