I have noticed on several occasions that when attacking a planet often a second attack on a planet does not always deal damage.
The population will remain the same but I get XP for the hit and it reports the normal (-20).
I am not sure if it matter but I normally notice when doing the Secure Surrounding Space, but that is normally when I am looking at the pop so it may be irrelevant.
I am also dealing max (20) damage not sure if that matters either.
I have also noticed that as far as I can tell it always seems to be the second attack that is not registered but the first and third onward seem to be fine.
Below are images of 3 back to back attacks on a planet with a few second between hits for screen caps.