What is your soil mix (what's in it)? How often are you watering and how (from above til it comes out the drain hole, or by immersing the whole thing in the sink)? How much sun/light is it getting for how long every day?

The tree is in commercial pre-packaged bonsai mix. It's very porous with some gravel & maybe charcol. I've been watering every 2-3 days from the top until it comes out the drain hole. I have it on a light stand with plain 20 watt tubes that are on for 14 hrs/day.

Well, the light is not nearly strong enough plus the tree should be outdoors in any case, and I would definitely cut back on watering frequency. The mix you have may be ok, but many pre-done ones are not, and it's always best to make your own if you can, or at least add a fair amount of perlite to the mix. And once it's outside (in the ground might even be best for now) give it a few weeks to recover.

I have an older Calliandra tweedii. They need a tremendous amount of light. Less than full sun or metal halide and they will stretch out long branches with nodes far between.
They arent fussy on humidity,but resent root pruning taking quite awhile to recover. Even in full sun i might add,they need frequent pinching to keep compact,although they never seem to get as dense as Bonsai Acacia's. And all that pruning really reduces it to a foliage plant,flowering only at the height of summer rather than than the all summer long flowering of an in ground plant left to itself.