Jack Bros Glitch?

I’ve noticed something odd (and irritating) when playing Jack Bros. At some point during a play session, I’ve lost the ability to hold down an attack button and have the character continually attack. I’m reduced to button mashing to keep up my attacks. Is this a glitch, or is there some button combo that disables the ability to use sustained attacks that I may have inadvertently pressed?

The first time I came across this, I was playing on an emulator (using debug cheats) and I assumed it just went haywire for some reason. However, it’s happened more than once since then when playing my cart on the real hardware (US version).

Anyone else have this happen?