Our QA has been reporting this issue and discovered its an issue with Juicebox gallery affecting the fullscreen functionality of the YouTube video players in the same page.

I factored out that issue that any javascript code introduced in our pages is causing it.
I did that by embeding a youtube player in any of the Juicebox gallery pages (ie https://juicebox.net/demos/pro/embedded/#) and running similar steps below.

This occurs in the Android devices that our QA team tested but this can be reproduced also using Chrome's simulator for mobile devices
1) Open Chrome > Tools > More Tools > Developer Tools
2) Click 'Toogle Device Toolbar' icon (top left of developer tools)
3) Select 'Nexus 7'
4) Visit https://www.warnerbros.com/justice-league/photos and click a thumb to expand gallery (or go directly to https://www.warnerbros.com/justice-leag … #expanded)
4) Exit fullscreen
5) Scroll up and play video player
6) Click fullscreen in video player controls (left most button)

Actual Result: Video player switches to fullscreen for a second and minimizes.