I'm unclear about a few of the facts here.

1. In the beginning of your post, you say that the tree broke at the graft, but then later go on to describe the break relative to the location of various supports. Was it the actual graft that failed or did the scion or rootstock actually break in two somewhere. Pictures might really help here. I'd also like to see how/where the tree was supported.

2. You say that the tree is limp and leans. Is the entire tree shifting in the ground or is the wood of the main trunk bending? Here again a picture might provide a better feel for what's happening.

The fact that the tree contains multiple cultivars won't cause it to lean, break, or become limp. Poor grafting technique might be to blame (depending on what the details/facts turn out to be).

It could be a poor graft, how old is it? Even so, any top grafted trees special attention is needed for a couple of years until the graft is almost invisible, a heavy load of apples can break it right there,...especially when these branches weren't pruned properly. I don't think root stock is the cause of this problem.

Konrad

Maybe the graft broke because of wind, if both the trunk was staked and the Fuji branch was staked. It could put too much pressure on the graft union with a strong wind and nowhere else for the tree to bend.

I also had a 3-year-old graft break at the union, but it was because I didn't thin enough and let too many fruit grow and it couldn't hold it.

Carla in Sac

I don't have any answers as I have never purchased a multi-grafted tree. I just wanted to point out that I have heard about this happening many times.

I don't think I will be buying a multi-grafted tree anytime soon.