Im using the scroll to trigger scenes transitions, but Im trying to use a background as persistent symbol so in that way the background stays the same or I can modify the content via “custom behaviors”.
I have reduced your file down to the bare minimum - just the symbol - set to persistent - with the enclosed image; all code and ancillary files are removed. A mouse click on the image to change scenes.
The symbol disappears as the background is transitioning (what ever the transition style) in the four major browsers; however, if the symbol type is set to standard all works as expected.
I have also started completely from scratch. As soon as the new symbol is converted to persistent, the result is as before.
Unless there is a compelling reason to have the symbol be persistent, I believe using a “standard” symbol instead works just as You wanted to start with. Note: In the version below I removed the video sequence to save space, otherwise it is just as your original file.
Without being the background as persistent, the background, even if its the same, changes / moves with the transitions, its disorienting and cause eye straining.