Having negative pressure in incompressible simulations is OK as we are working with pressure differences. In such simulations, you can add any constant to your pressure field and values are still relevant.
By the way, we recently moved to a uniform pressure unit in all simulations. Therefore, even in incompressible, Pascal (Pa) is the unit. We added the term gauge pressure as well to hint that any surrounding pressure, e.g. atmospheric pressure, is not relevant. Having a zero or negative value for gauge pressure is not incorrect in itself.