You have done your homework well in tracking this down to what’s known as hollow heart. This is a sugar concentration that sometimes forms in the potato during the growth period, and is undetectable until use. Many factors contribute to this condition. While hollow heart is an annoyance and can very occasionally arise in any potato variety, it not harmful. Just scoop out the affected part of the potato, then use as desired. Here’s a link explaining this more: https://idahopotato.com/dr-potato/there-is-a-dark-hollow-pocket-in-the-middle-of-my-potato-what-is-it.

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