I mean you have to manually specify the animation center for each of your characters animations - usually placed near the feet. It is used to make sure all of your characters animations are aligned so the character doesn't appear to jump around. If you haven't set the animation center for some of the animations then it could appear the character is disappearing, but is in fact getting the animation pushed outside of the current viewport.
But what you are saying is that sometimes you can see them walking & other times they just disappear? Is anything mentioned in the log?
Yes, the animation center is set for all animations. The prove is that I usually see them correctly working.
Yes, sometimes I can see them standing but the walking animation disappear. Or sometimes is the standing animation that disappear but the walking animation is working. Or sometimes they just stand and they simply blink. Nothing mentioned in the log. It's very frustrating because I have no control on this error.