Sorry for being late. I have a small paragraph from a book of mine that talks about this. Its in my home language so I will try to translate.

"The other peculiarity of the salient is the "Kursk magnetic anomally" caused by an underground mineral vein of iron situated between 250 and 500 meters underground. This vein is closer to the surface at some places and in Zheleznogorsk there are open pit mines.

It was a team of german cientsits that, at te begining of the 20th century discovered the presence of iron that has a distorting effect on compasses. However both the germans and the soviets were fully awere of it, and if there were any navigation errors, it is unlikely that the anomaly was responsable."

Does nobody know? Or is this not really an issue?