According to this article by Spiegel from 1958 it seems to have been a hoax.

As the Polish authorities did not comment on the reports, the press office of the Warsaw Ministry of the Interior commented: "The case is not known to us at all"

This NYT article claims the soldier died 5 days after the first reports from scurvy. I am not a doctor by any means, but surviving on a vitamin C deficient diet for 6 years and then dying of it not even a week after being rescued sounds a bit odd to me.

It might be true, it might be not. There are many parts of the story that sound a wee bit too fantastic to me to be true, but without any further sources it's hard to come to a satisfying conclusion.

This was an event I had read about in passing and tried to discover any factual basis for it. Unfortunately, all I could find were circular references which all referred to each other.

However, tellingly there were two movies released which incorporated elements of the news report. One was The Blockhouse and the other was Nasser Asphalt or Wet Asphalt, a thriller produced in West Germany.

The Blockhouse is more about what happens when people are confined. However, the entire plot of Nasser Asphalt is that the story of the entombed soldiers is an invention of a journalist desperate for a story.

It was released in 1958, just seven years after the purported event. It would seem a mite insensitive for a German director to use something as traumatic as a 6 year imprisonment of German soldiers as a plot point if that story were genuine.