Ok here is a really funny bug report. It took me a while, but I found the item interaction that is causing it.

I have Solomorne who is wearing the High Spinner chest armor. This chest armor causes adjacent enemies to make a Willpower test when they enter his proximity.
He also has the "Public Sentencing" arbitrator origin talent, which causes him to make an attack of opportunity whenever an adjacent enemy fails a Willpower test.
These two items have an obvious synergy and I paired them on purpose, but the bug that results from them together is hilarious.
The third piece of the puzzle is the "Prepared Prosecution" arbitrator origin talent, which allows you place Solomorne next to an enemy during the tactical phase, the pre-fight placement phase.
During the pre-combat round where you place your units on the grid to determine where they start the combat, you can actually move Solomorne next to an enemy during this placement round, which causes enemies to make the Willpower test from the Spinner chest armor, and he will actually shoot them during this placement round if they fail it. The funny part is that you can keep doing it endlessly, since you can move your guys around however often as you want during this round. Which means Solormorne can actually shoot every single enemy with his shotgun for free endlessly before the fight even starts. Here is a screenshot of every single enemy in this encounter already half health and on fire during the placement pre-fight round because he shot every single one of them. This is one of the funniest bugs I've encountered.