Is there a problem with Inflation-corrected real price?
You are right that if the inflation rate is calcualted using PPI, then PPI-adjusted price for copper didn't not change, i.e. the real prie for copper did not change. However, we would also consider the real income and real prices of the necessities in determinging wether copper has increased in value reative to income and daily necessities. Since this time when we compute the real income, we would be using the PPI calculated inflation rate as well. If nomial income rose by $5\%$ by the end of the fifth year while the price of copper along with other PPI goods rose by $15\%$ at that time, we would get a real income that is lower than the real income five years ago, which then would be apparent that the value of copper, or the relative value of copper, has increased.