Cal/Mag?

judging by watering pattern, could be irrigation related habits. They weren't in solo cups long enough to be root bound, but that doesn't mean the balance of any fertilization applied wasn't off. You may want an intermediate size pot before you jump to a 5 gallon. A tiny plant in a small pot is difficult to water. Make sure you get teh water soaked in all the way to the bottom, even if you do a smaller circle around the small plant... increasing size of watering area as the plant grows. this inherently adds risk of root zone issues. if soilless, get 10% runoff, too. Mg would come with interveinal chlorosis with rusty spots and it is mobile, so it should start at the bottom. It is in the right location but the symptoms don't match. Calcium is yellow-brown rusty spots that would start nearer the top as it is less mobile. You use foliar sprays? potentially related to that. This spot is in the wrong location. Probably rootzone related from overwatering or not fully watering - or - something dripped on the plant, like a overly concentrated foliar spray. foliar sprays are completely unecessary, so it'd be wise to stop doing that anyway. they are fine for a specific problem, but you should provide nutes properly to the roots and not ever need one for that sort of thing.