I do realize it can difficult to follow an active game of Espionage being played by two other players, not being easily able tell which pieces are which, but if you do look through the several moves since he’s had only two pieces left, and otherwise think a little more about the situation, I think you’ll realize that the only winning strategy does require the following of the rule that is broken by this bug.
My thinking is this: My opponent’s best moves, without following this rule, are to constantly move his pieces around in a tight square at b1–c2. The only way I can win if my opponent does this is to place one piece following his 2 until it can capture the HQ. With this bug, my opponent will be able to capture my piece whenever I try that.
I’ll consider messaging Fencer, but I would have expected the bug tracker to be the best place to report this to Fencer.