So I've found an alien structure and investigated. I set an attack on arachnids with 0 power, and opening fire results with a massage saying "No defender units!". The attack remains stuck in open fire phase
After investigation, all alien structures always have defenders on it. The mere fact that you said that you investigated it and that the arachnids had 0 power is already something unusual. LOL
Based on the pic, it's really a bug, or maybe you got the wrong planet and initiated (refreshed) the attack there.