I've been using the physics capabilities quite a bit in Char Anim. I absolutely love it. There are a couple of quirks that are difficult to work around, however.
I've been creating Rube Goldberg machines using physics. It's quite realistic for a software simulation. I have about 20 balls on small perches, with one ball above them that comes sailing down a slide. I painstakingly moved all the balls around and played with the bounciness setting until I got all of the balls to be hit off of their perches. They'd fall and bounce at the bottom, then settle in. Set up something like this in real life, and the balls would come to rest in a different order guaranteed. With CA, the physics is just math and it computes the same every time. When I refresh the scene, the balls are always all hit off their perches and always bounce around and come to rest in the same order. (I carefully added letters to the balls so they would spell out a message, awesome.)
Then I added some artful stuff to the upper corners of the scene, nowhere near the bouncing balls. Suddenly the physics changed, with no real reason as to why. I did not give the artwork Collide or Dynamic properties. I had to look into it further.
In a different scene, I had a ball rolling off of a ramp and needed it to land in a particular spot. I adjusted the ramp artwork in PS to try and dial it in. By adding just a few pixels to the end of the ramp, it changed the way other items in the scene reacted--even objects nowhere near the ramp. By adding to the ramp to make it look better, but without affecting the slope or length of the ramp, suddenly the ball would react as if it were heavier and had lost its bounciness. In other words, there are times when I make insignificant changes to elements in the scene that should have no effect on the physics, and it has a tragically huge effect on the physics.
I realize this must not have been an easy piece of software for the engineers to put together. Is there any way I can gain insight into some of the nuts and bolts of the programming that might be causing this? As a former programmer, it sure would help with my animations.
I've asked the developers - I'll let you know if I hear anything back (or they may respond here as well).
Because of the nature of simulation, tiny differences in the setup can product vastly different results, especially when there are collisions happening. That's why playing pool is hard 🙂
That said, if you are only changing things that aren't set to Collide or Dynamic then they shouldn't affect the simulation, so that may be a bug. If you privately DM me your project (with steps to reproduce the surprising situation) we'll take a look.
If you click on my user name, you'll see a Send a Message button to send a private message. You can then send a link to the zipped project folder (dropbox/googledrive/adobefiles etc.)