Unfortunately, I think that there is probably little that can be done.
It sounds from your post that the Messenger app has its own internal browser which does not render the Juicebox gallery correctly. (I do not use the Messenger app myself so I could be wrong about this.)
If this is the case, then I think it is unlikely that the developers will modify Juicebox to work within the Messenger app.
The only mobile browsers that are supported are Mobile Chrome and Mobile Safari.
However, just in case the problem can be resolved by upgrading software, please make sure that your versions of iOS, Messenger and Juicebox are all up-to-date. Current versions are:
Also, please try with a link to one of our own demo galleries to see if the same thing happens (to see if the problem is with all Juicebox galleries or somehow unique to your own... gallery dimensions may play a part in the problem somehow). Try this one: https://www.juicebox.net/demos/pro/default/
Please also post the link to your gallery's web page so that I can take a look at the gallery live on your web server.
Lastly, it might help to see what you are seeing so please post a screenshot of the problem so that I can see what you describe.
I really do not know if anything can be done but with a little more information, I'll be happy to notify the developers and let them know about the issue. Thank you.