While I have only encountered this once, it is safe to say that it would apply to every encounter between these creatures due to their AI. I do not know why the Gnat Swarm got aggro'd towards the Thunderbird, I just happened to find them locked in a battle. The Thunderbird does not summon lightning to attack the Gnat Swarm, and the Gnat Swarm cannot properly harm the Thunderbird, causing an infinite loop of the Gnat Swarm wandering slightly off and then snapping back to try and attack the Thunderbird once more.
Steps to Reproduce
If the AI is the issue, coaxing a Gnat Swarm to attack a Thunderbird should be all that is necessary to reproduce the bug.
If the AI is the issue, coaxing a Gnat Swarm to attack a Thunderbird should be all that is necessary to reproduce the bug.