First, the original issue:
The hip assembly on Optimus Prime and Motormaster don't allow Menasor and a Combiner using the Optimus Prime torso to stand straight legged when the hip panels are locked into place. You can see that Optimus Maximus, on the right, is able to stand in a straight legged pose with no issues, even with the hip panels locked in.
Here I've illustrated the same thing with just Menasor and Optimus Maximus.
Attempting to bend Menasors hip inwards, while leaving the hip lock intact, leads to imbalance as well. Battle Core Optimus Prime also holds on to Legends Rodimus much more securely than Motormaster or Optimus Prime can, though I can't find any obvious engineering reason for it.
If we look at just the torsos, it highlights the issue with the initial runs of this mold. And yes, I need to clean my table. I'll bring my dog over to your living room and we'll see how long yours stays clean
The problem here clearly comes down to the hips of the Voyager torso itself. This mold has angled feet, so the individual robot mode really wasn't intended to stand hips-squared, but the original Optimus Prime usage of it and the subsequent Motormaster release both allow this.
Upon opening up the hips, you can see this pretty clearly. This is the default position of the hip ratchet when you open the figures, with Battle Core Optimus Prime on the left and the original Optimus on the right. It isn't extremely obvious here what they did to fix it. They couldn't have just changed the position, because then mere clicks one way or another on the original mold would solve the problem. What did they do?
See it yet? Don't count teeth, it won't help.
As you can see in the pictures above, this single piece is the change. By shifting the gear teeth by half a tooth, the Combiner that you can build with the figure can now give you a tall, proud, standing-up-straight pose.
With Motormaster scheduled to be re-released in Wave 4, will a running change be made to him as well to allow for sturdier Menasors? You know, there's hope in not knowing
If you do get a Motormaster out of this wave's case assortment, give it a look and be sure to let our News Team know if this fix has been applied to his re-release as well.
to be had; I would however get the ratchet mold because the first-aid mold makes for exceptional limbs 
Oh goody then! Sometimes it pays to wait.