Potatoes, one must keep in mind are living organisms. As such, potatoes are subject to minute nuances. Most potatoes perform when cooking as expected, with few surprises, and most performance-related issues have more to do with storage and handling. However, on very rare occasions some potato varieties can produce an overabundance of solids. These overachiever supercells, when prepared, may not break down chemically during the cooking process, and the result (while again very, very rare) can occur. Here is another post on the topic:  https://idahopotato.com/dr-potato/potatoes-that-will-not-cook