I think the issue is that pico8 doesn’t do any debouncing for you, so mouse button presses are not aligned to game loop frames. You would need to keep track of the button state yourself to detect if it’s newly pressed in this frame or newly released.
Stat reference: https://pico-8.fandom.com/wiki/Stat#.7B30.E2.80.A636.7D_Mouse_and_keyboard_in_.22devkit_mode.22