Readers Left Hanging?

A quick AI on this questions resolves to: “Good novels generally do not leave readers hanging at the end – they provide a satisfying, complete narrative arc. While cliffhangers are useful in series to build anticipation, a good book should resolve its main plot, avoiding the feeling that the author just arbitrarily stopped writing.” And, I mostly agree, unless a novel is part of a series. However, I want to add a small disclaimer. In MUTINY, there is one glaring and unresolved plot element (in Chapter Two) that is not resolved – one the occurs early on and is briefly revisited by the lead protagonist. This element does not get answers and the reader may well be wondering “who done it”. Was this deliberate or did I just not work out a solution? Or, did I plan it this way to open the door to a sequel? Aha, there you have it, dear reader and may the mystery continue for now.