Hey, I have the similar issue - Android 6.0.1, Resilio mobile 2.4.9 (639)
I've enabled sleep mode and check to sync every 4 hours, or every 5 mins while charging.
Have a backup of my DCIM folder on SD card to my other machines.
So if I check those backup folders on other machines - I see only some photos which was made 3-4 days ago, not even those which was made earlier that 4 hours ago as in settings.
But the craziest thing is that Resilio mobile can't see some files right in my phone!
For example, I have 5 files in DCIM folder in my phone, while I open Recilio on the same phone it tells me that I have only 4 files in that folder, and it never backups that missed file of course too.
Usual folders (not backups) works like a charm, the problems only with Backups.