Seems like it's a bug with the game?
After zero-ing out that address, it does
038F ld a,$01 3E 01
0391 ld ($F68D),a 32 8D F6
Then reads from that address and thus gives 1 credit.
EDIT: nvm. For some reason, the Z flag gets set on the main Z80 the third time it gets to
where it then goes through a few subroutines, the first of which is the one mentioned above. This doesn't look to be an MCU issue, unless the MCU can affect the flags on the main CPU.
EDIT2: Well I'll be darned.
- gladiatr and clones start with one credit due to the way MAME initialises
memory and the dodgy code the bootleg MCUs use to synchronise with the host
CPUs. On an F3 reset they randomly start with one credit or no credits.