Is there a way to force Google Photos to upload in the background?
Simply make sure the app refresh feature is enabled for Google Photos, enable auto-sync and when you're on wifi AND your phone is plugged in to a power source...it will continue to backup. Main issue people have is not having your iPhone plugged in. :)
So I was experiencing the same issue and turning off background app refresh in for Google Photos under the settings app and then turning it back on seemed to do the trick for me.
Also when phone is in power saving mode background upload also disabled.