I'm concluding this is a hardware problem. The design of the tactile switches of the Playdate's buttons just (apparently) won't allow a new press to be registered before the previous is finished (i.e. the switch goes back up). Gamepad buttons with conductive pads or the switches used in Nintendo systems (for instance) are better at taking ultra-rapid changes in presses, and I finally used Mirror to confirm this for myself.
Apologies for kind of going in circles on this till now, but Dave, thanks for your assistance and SDK work. This tiny game can just get too intense!