While I can't really help with bypassing it, I do have a hunch as to why you've gotten larger payouts before without hitting a jackpot lock-up and needing to use the key. I have a Bally E 2214-38, and the front panel has a notice that reads "200 coins paid by machine when jackpot is 600 coins or more - balance paid by attendant" (picture attached). It's still possible to get 200 - 599 coins paid out by the machine on a smaller win (or 500 at least, that's the highest sub-600 value on the pay table), but if you hit 600+, you get only 200 out. Why this design decision was made is beyond my knowledge, but it sounds like something similar is at play on your own machine- with the jackpot threshold being somewhere at or below 500, but above 250.