It's not particularly complicated, it's simply a feature of the platform to preserve battery life and other computing resources on client devices. It's also an old issue that I understood to be resolved, as Chrome now has an exception for WebRTC connections and keeps the tab active.
However if you're using Photon that might not use WebRTC or websockets, so either slap the multiplayer plugin into your project to keep a "fake" connection alive, or play some silent music, which also bypasses Chrome's out of focus throttling/suspending IIRC.
Not familiar with Photon though, so it may or may not work. You'll find limited support for third party plugins on these forums.