The term is Fugitive Ink. Inks which are designed to alter in the presence of water or other chemical. For example ii you pick correctly, you can watch the color float of the stamp in watermarking fluid for stamps of certain printing methods.
As to debating if I have or you have a better experience defined percentage I will point out using some general figures. Since 1840 when postage stamps were first issued, there have been well over 200 different "countries" issuing stamps. I will use 200. In general an average country has issued over 1000 different stamps. But I will pick 1000. That means 200 x 1000 = 200,000 different stamps. Stamps include all government issued or government approved stamps, postage and fiscal.
No one on this forum has soak all of the 200,000 different stamps. Soaked 1000 or 10,000 of one stamp or of a "type of stamp" British Machins or US Washington-Franklins being a type example is not uncommon. Yet if you are soaking modern self adhesive (aka pressure sensitive adhesive) you will find most will lose some amount of ink. Likewise if you soak only fiscal/revenue stamps you will find a larger percentage of stamps which are altered by water.
Now all of the above is directed at ONLY unused stamps. For used stamps, the method of cancellation matters. Often postmarking (cancellation of value) introduces inks which also responds to water soaking usually by fading or bleeding. So if there are just 5 types of inks used for cancellation to demonetize the stamp, of course there are many more than 5 but I will use 5.
That means after we have the 200,000 different stamps mentioned above each with now five different cancellation inks we get 200,000 x 6 (1 uncancelled and 5 cancelled) = 1,200,000 different stamp and cancellation numbers. Again now one on this forum, nor off of this forum has soak enough stamps to have a definitive percentage of the total, just some tiny number from their very limited soaking experience. That is why,
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To give an exact percentage is impossible. No matter what you say, it will be contradicted by another person's experience, making it conjecture
is completely correct.
For Ringo I add the reminder that stamp printing ink applied is not always for the design of the stamp but the the light reactivity of the stamp when exposed to certain wavelengths. Are you checking what happens to those inks upon your soaking? I think not.
All the above was about ink. Remember the coated papers used as some stamp printing paper also can affect how the stamp responses to soaking in water or other fluids.