Use the part number to look up the datasheet. Chip Quick makes several kinds of solder with several kinds of fluxes.
What you have there could be no clean, no clean water soluble (which as you might guess must be washed with water), or rosin.
If the board MUST be washed after assembly, try to choose a flux that is designed to be washed off. Some of the no clean residues are optimized for not being cleaned and will not as easily be removed.
Rosin and many synthetic flux resins are soluble in ethanol, but the tiny bits of salts leftover from flux turning copper/tin oxides into ions and/or from many inorganic acids which may be added will not be soluble in ethanol. It may be sufficient to add small ratio of water to the ethanol. If your standards are high enough, you might need to do multiple baths... ethanol, then deionized water (plus maybe degreaser/detergent). And maybe finish with alcohol, again, just for drying.
I still have bad leakage, and it gets 10X worse when it is humid here.
Any bare salt left on the board will increase in conductivity with humidity, which can happen with improper cleaning of any flux residue. With enough water, salt dissolves into conductive ions. But this is esp noticeable if you are using a flux with a water soluble residue (which you have not completely removed). Depending on the application, improper/incomplete cleaning is often worse than not cleaning. Taking a close look around component leads under the microscope can give you another clue as to what you're dealing with. Dry tin/copper salts are white.