*very* nice plant! Amazing how fast plants can grow when they are in the right climate. Tree seedlings here in the Netherlands hardly ever reach that size in one year. *sigh*
Hm.. Have you been fertilizing it? I am slightly worried that the plat is getting to a phase where it is looking for the fungi it needs for symbiosis. But I am by no means an accacia expert. Have you considered calling some of the growers in your region? This can also just be the natural aging of the branches, where leaves closer to the stem make place for more extended foliage. If you can get to a spot where these trees occur naturally, you could have a look around those and see what they do now?
Snipping the taproot can cause the plant to stop growing. You might find when you repot them next year that the one that did not grow above-ground, has a nicely developed rootmass. It may even get a jump next year and surprize you. With seedlings I have been growing I also noticed such variety (
www.bonsaiempire.com/forum/basic-techniq...bonsai-a-pre-article).