So slowly adding water makes no change, at all, in the level?
Wicking was one thing that wasn't mentioned. Primarily because you didn't have the symptoms of a slowly receding water level. Can be referred to as capillary action. When you have something like toilet paper, piece of cloth, strings, etc. hanger over the trap weir and drawing water over it.
No! It's just another way to get the bowl water to be lower than the trap weir. Thought I should mention it in case your level went up a bit when adding water but then went back down to the low point.
But you're sticking with the no change in that low level when adding water. I believe you but can't understand it.
I don't remember if this was mentioned but I would ask the mfr if they had any thoughts.
That certainly sounds like the case except the water surface area is 5" x 4", which is consistent with OP's statement that it is "very low".
The only thing left in my mind is something that may not be likely, if even possible with this particular toilet, and that's an internal crack large enough not to let the level rise and then fall very slowly.
I called and was on the phone for close to 10 minutes with American Standard. They kept telling me that they have steps they would talk you through to try to raise the level. When I told them that we added water directly into the bowl and it didn't rise and slowly go back to the 5"x4", the person insisted on going back to there normal trouble shooting method.
The person did say that they have seen internal leaks that resulted with water on the floor as well not showing up and going down the drain.
Hopefully the water was added very slowly, like a cup at a time, or so.
EDIT: The only reason I called them was to find out if they had a cross section of the trap way I could get to see. They said no. I don't believe they don't have it but I guess not that I can see.
You could run a camera through the trap and confirm the water is at the level of the weir. I wonder how much higher the level would have to be to show a "correct" area? if it ain't much, a damaged or mis-cast weir would account for the lower level, and, again, a camera would show that.