Based on your description of the bug it feels too "natural" to be deliberately inserted into the code.

An unnatural bug would be something like the Konami code from video games, where you have to input a long sequence of inputs that has no logical consequence or meaning.

In this case you can immediately "get" why the bug works in terms of operational logic - the DU question leaves the hand unresolved but also leaves you with no meaningful inputs, *except* you can cancel out of the question without answering it by inserting a bill because the whole "if bill is inserted, change machine state to free" loop was not blocked off during the DU? state. From there you pick the new game and cash out which apparently inititiates the "resolve hand and settle up" loop.

If it's malicious it's rather elegantly so. A blatantly malicious bug would be something like hitting a random string of 100 hold button presses and the machine then gives you a royal. Something that would never occur due to a breakdown in program flow.