It looks like something is setting a height for jp-carousel-overlay
independently of the Jetpack plugin.
The overlay dimensions are set here (note the absence of height:
being set in the code): https://github.com/Automattic/jetpack/blob/master/modules/carousel/jetpack-carousel.js#L67
That means there might be something in another plugin or your theme setting a height either for the gallery or the page in general and the setting is being inherited.
Can you try these steps to troubleshoot your problem?
1. Disable all other plugins, then check the slideshow to see if it still scrolls. If the scroll is gone, turn your plugins back on one-by-one until you start seeing the scroll again. Then note the plugin that caused this error and get in touch with us. Sometimes Jetpack and other plugins are incompatible; just let us know and we’ll see what we can do.
2. If that doesn’t help, try activating Twenty Fourteen temporarily as your theme. Then check again. If it starts working, likely something in your theme is conflicting and you should get in touch with your theme’s author.
If you’re still having the problem after following these steps, please let me know and I’ll be happy to look into this further for you.