Yeah, the dynamic posing of several AR models suspended in mid-air on a tiny connecting point will do that, I can imagine.
Zreef, who posted just above, later started a thread called "the Leo sag" about that exact issue you describe: viewtopic.php?f=5&t=33968
I think the best thing that can be done to prevent this, that doesn't involve propping him up, is doing what Gomez (and zreef is his sag thread) describes about drilling into the leg through the base and getting some sturdy paper clip wire as far in there as possible. The resin itself is soft and will bend, so you need something hard and metallic to act as a skeleton inside the leg to keep it in position.
Of course, the easiest way is to just prop him up on a stick or a rock or something. But that won't look as nice as him being actually in a mid-air leap.
I have a Zahra myself where I haven't yet seen a problem with sagging, just with breaking. I hope she doesn't sag.