When I tried to add multiple pictures in the gallery block, I got a JS error:
Uncaught (in promise) TypeError: Cannot read property 'large' of undefined
To be more precise, when click on Media Library, it’s working, not when using the Upload button.
If I check the content of the newImages variable, I can indeed see that it does only take the first selected picture.
When editing the gallery, it does include all pictures from my media library to the gallery.