Could be inadequate lighting, pest or alkaline soil locking the iron out making it unavailable to the plant so it develops iron chlorosis which is characterized by the leaves of the plant turning yellow/white, but the veins of the leaves stay green. Usually leaf chlorosis starts at the tips of new growth in the plant and will eventually work its way to older leaves of the plant as the deficiency gets worse, followed by leaf loss and stunned growth.
I highly doubt that its nitrogen deficiency because it always starts at the bottom/older leaves of plants because they suck the nutrients from the older growth to support new growth. And the problem you are experiencing with your mimosa starts from the upper growth so i highly doubt its N deficiency. However it might be combination of factors that caused your Mimosa to be that way. Too high soil pH, too much clay/sand, too compacted wet soil, too much P, pest, too little exposure to direct sun etc. It doesn't look like pest or sun damage.
How to fix your problem: improve soil texture, drainage, if your soil lacks organic matter and it has too much clay or sand and its too compact or its just cheap poor soil you can add something to fix it like perlite, manure, compost, worm casting etc. on top and mix them carefully within the top few cm of the soil and if its too compact you can carefully loose it up a little. Dont over/under water, dont push hard the growth accelarator and give it time to establish itself, while small, plants tend to grow more underground rather than above. Check and fix soil pH, chances are the problem is the soi pH (too high) or its fertility (lacking/locked N,Fe) if you are giving wrong liquid fertilizers maybe the problem is there, give it more N instead of P or K (too much P can cause this). Consider Fe rich foliar feeding after the sunset so you dont burn it.
Increase the direct light exposure GRADUALLY so you dont kill it accidentally from shock.
For me to give better advice i would need more information.
Good Luck
