GUARDIAN BUG!
I already posted on official Facebook page, but will put here too.
This same issue extends across all of the Facebook servers. Multiple people have been reporting missing reports and energy.
Whenever a player sends out multiple guardians at the same time to attack warlords, sometimes they find that they are missing an attack report and one of their guardians has no extra XP despite only sending to weaker warlords.
Today I've tested what I thought it might be and I found the conditions that cause this problem. When you send 2 guardians after the same warlord, only the first one that defeats it will get XP and show as returning to your city. The second will just suddenly end up back in your city, no report, no XP and will have used up 10 of your energy.
This can't be an intentional part of the game.