Proof dies strike about 2500 to 5000 coins before they are retired, compared to +/- 250,000 for non proof coins, so it seems unlikely to have signs of deterioration.
It could just be the result of the multiple strikes that each proof gets.
On the other hand, VV recognizes at least 15
DDO's for 1970 S Proofs, so you may want someone else to look at this.