Seems like a TD would need to have great knowledge of the game’s rules to make this call (switch malfunction) to rule out any other possible explanations of how disc multiball started. Doesn’t Flynn’s Arcade sometimes award “advance disc”, so it’s possible the player started disc multiball this way?
Is the stream available online to review?
Seems like if the switch (opto?) by the disc triggered erroneously the display would still show the animation indicating a disc hit (X more to access disc), and if the player was 2-3 shots away there would need to be 1 or 2 of these animations before the final switch hit to start multiball.