What you are describing can happen more frequently with some medium solids (starch) varieties, such as the Russet Norkotah. It typically is limited to the core of the potato when it occurs, sometimes referred to as the stem. During the growing season, unusually cold weather in a field may cause the potato matter to remain dense thru the length of the center of the potato. When baking, I have actually had it become stringy and resist even cooking it out in the microwave. Had not heard of any issues this crop year with that variety, but sounds like what you ran into.

Check the bag plastic quick lock, all Idaho potatoes have to list the variety. The Russet Norkotah is an early harvest variety; it comes on board in August to September and the storage is usually finished by February or March. Here are some details on what it looks like.

Because of its nice even skin and oval appearance the Norkotah is quite popular with retailers. The Russet Burbank variety is what Idaho is typically known for, and comprises most of the production. It is used extensively in foodservice. Here is a link describing it.

All Idaho potatoes sold in bags are also required to display the “grown in Idaho” seal, so also look for this too.