When adding a scene at the end of a video, the prior scene is copied with characters and assets placed in the exact position as they were at the end of the prior scene. All elements are copied except speech bubbles. This is by design. It has been designed this way so it's easy to create a dialog.
However, if you want the speech bubbles to remain in the scene here are some alternatives:
Option #1: Copy the scene and insert it.
Use the Copy Scene option to make an exact copy of a prior scene (including speech bubbles)
and then Insert after the current scene, as shown below
This results in a new scene that looks like this:
Option #2: Use a prop as speech bubble.
Instead of using one of the speech bubbles among text boxes, you can insert a prop and add text separately.
On the Props library, select one of the speech bubbles available and add it to your scene.
Then add a text box and type the dialog.
You can add more scenes and the speech bubble will remain in place.