When one purchases items from any of the giant online retailers, one has to assume that due-diligence is part of the total cost when purchasing those items.
That applies to almost everything...... I.e. a power supply rated for 50 watts, may only deliver those 50 watts within a very narrow input voltage range.
This "feature" won't be listed, this forum and others are filled with stories of unsuspecting buyers where the device they purchased fails to meet a particular feature.
Returning to the case of the solder, there are two clear paths to follow:
-Perform the due diligence yourself by employing some test coupons, as suggested by Helius
-Or assume that you should always clean the flux residue. And continue to do so.