I noticed that they always deteriorate when the third set of leaves are forming. If these are in fact stunted from the nutrient lock out due to hot soil, does it make sense that growth is vigorous until the 3rd set of leaves form?
Its burn, not lockout. Rather, its locked out due to an early burn, if the soil was indeed hot.
Re-pot em in proper ph and low nute medium. They will erupt once cured