If water is coming out of the vac exhaust, it is because there is water in the vac impellar housing. If it has been sucked in from the waste tank, it will be dirty water coming out.
As the vacs inlet hose is attached to the waste tank, that's the only way water can get inside it.
When I was a porty user my last machines safety waste water full level, cut out, stopped working and waste water would often blow through the vac exhaust onto a strategically placed drip mat.
Dave.