Reinforcements can go invisible sometimes. I'm basically certain that it's related to the Forest Keepers' Circle of Life, maybe if it activates during the death animation? It's really annoying to reproduce, but it will pop up when playing with Forest Keepers somewhat often.
https://cdn.discordapp.com/attachments/423213564644556822/786284676867555328/Invisible_reinforcements.mp4 https://cdn.discordapp.com/attachments/423213564644556822/768224155334082570/Screenshot_20200701-130003.png
Reinforcements can go invisible sometimes. I'm basically certain that it's related to the Forest Keepers' Circle of Life, maybe if it activates during the death animation? It's really annoying to reproduce, but it will pop up when playing with Forest Keepers somewhat often.