Video should be self-explanatory. Very tight timing on the activation of Inferno Brigade, if too early or too late the attack will fizzle and only Parasoul will shoot, resulting in only 10 hits (instead of the usual 60 or so)
I managed to reproduce this bug on my end and can confirm it affects the iOS version of the game as well.
Furthermore, if the timing is slight off, this "infinity brigade" won't appear and what ends up happening is Inferno Brigade doing a maximum of 10 shots. Sometimes it even manages to completely miss on a stationary opponent who just tagged in after their teammate was defeated. In this case the troops just do nothing but stand there for the remainder of the fight.