It's sort of hard to imagine this was a case of serendipity, for the planting, so I don't think it was broken on purpose or by chance. Initially, there were 2 plants, each with it's own trunk and root system. Where the trunks fused is an example of a process called inosculation. When done purposely, it would be called an approach graft. Here's an example of an approach graft used where a trident maple tree needed a branch where there was none, so I put one there.

I allowed a branch on the tree you see to grow long, and removed most of the leaves along the branch so it would be thin and flexible. I cut a groove in the tree to expose the cambium, then removed bark from the scion so the cambiums lined up. I then stapled the branch in place and covered it with grafting tape. The following spring, the stub was cut flush to the trunk.

This is a Ficus benjamina I started approach grafting about 20 years ago. Ain't she purty? Actually, I know it's very ugly and it's still a work in progress, but here it is after defoliating and wiring it last weekend:

No way to be sure w/o asking the grower, but I'm thinking the root system of one of the trees in your planting became infected with one of the fungi commonly grouped under the umbrella of damping off diseases. When it became evident one of the trees was infected, the grower noted the inosculation and reasoned rightly that the living trunk would support the part of the infected tree distal to (beyond) the point of inosculation, and severed the trunk.

It's not unusual for adventitious roots to form on trunks of Ficus when it's humid. Also, the flow of the growth regulator that stimulates rooting (auxin) is polar, meaning it only flows downward. When it gets to the bottom of the stub, it collects there and becomes a built-in rooting aid.

Though you can't tell, this ^^^ is a root-over-rock schefflera on which one of the mature roots died. The mass you see is what resulted - same thing you're seeing on your tree.

This ^^^ is the view from the opposite side.

You already know the living trunk is fully supporting what is essentially the approach graft or scion. The adventitious roots hanging in the air supply no resources to the graft at all. If your tree was my tree, I'd use a fine bladed saw (hack saw) and cut through the stub at the lowest point of fused tissue. I would then spritz the wound with cold water to stop the latex from oozing, clean up the wound with a razor knife so there are no fine threads of bark or sapwood tissue hanging from the wound site, dry it with paper towels, then paint over the wound with waterproof wood glue, being careful to cover the wound and any exposed cambium at the edge of the bark, but try not yo get any glue on the undamaged bark. I use the snipped off end of a zip tie as a glue spreader and it works like a charm. Painting over the wound decreases the size of the wound by eliminating desiccation and dieback of the bark at the wound site, and promotes rapid healing.

BTW - it would be best for the plant if you were to rid it of the support and the ties that are biting into the bark. They will soon be restricting flow of photosynthate (food) and other bio-compounds to the roots. While I see no visual evidence that the missing trunk was killed by a constriction, you might look things over and see if you think that could have been the cause.

Al