I doubt this is intentional but after filling up Octanes side of the field with Malignant Cells (I had a jade shield up to prevent his explosive gas) he kept putting up Eagle Eye's and then turning them into Flying Weapons.
The result? They simply disappeared. He did it twice in the game so I'm certain it's not a one off. I haven't tested it with non-malignant cell filled fields but I'd hope you'd get a warning about not being able to play the card if the field was full.
If not, then it's simply a bug in Octanes AI, although one that I don't mind he keeps