I'm having a hard time following your scenario...
How are you resizing your Stage?
In Edge resizing your Stage normally does not affect the size of the items on your stage.
I've run into the same issue. I have a symbol where the original stage size was set to small. I'm trying to lengthen it from 900px to 1200px but everythime I try to change the stage size using the width & height of the stage, everything on the stage get's resized, or "streatched" to the new height!
Right now I'm left with re-writing the symbol. Funny thing is, I created a new symbol, copied all the elements from the original symbol and when I past them into the new symbol (with the correct stage size) they still stretch out!
I have the exact same problem. I will say I have had compositions where I could change the stage size using the properties panel and it has no effect on the content on the stage.
I don't know what the difference is between some of my composition that this happens.
If I figure it out, I'll let you know.
OK, I've also been fighting with this but just found a solution!!!
Here's what I've discovered:
I have a symbol built to 900W X 200H. All the elements are set to percentages and many set to "stick" to the bottom right of the stage, so when I tried to increase the stage from 900W to 1280W, it all went out of wack position wise and everything "dragged" to the right leaving a "blank" rectangle to the left (but didn't leave everything in the same proportion of one element to another - some dragged more than others) (I have usually had to work from 1280w and then to 900w so everything was stuck to the bottom right. Now I'm doing it the opposite (MUCH EASIER!!!).
HERE'S WHAT I FOUND:
I changed every element to "stick" to the top left of the stage, then changed it all back to percentages like I had.
Now, when I stretch the stage out to 1280px, everything stretches to the right like I expect!
SO, it seems that if you stick all you elements to the "left" of the stage, then increase the stage width, you get what you expect. From there you can tweak things and change as you need!
I hope this helps someone out there - it has definitely helped me a lot!