Failed valve spring retainer (cup washer)

[v8nut]

I had my 24 T Fordor engine rebuilt about 2 or 3 years ago. I have put about 1000 miles on it since. Now, within the last 6 month I have had 2 cup washers fail. See pics below. I have been messing with model Ts for a long time and I have never seen this before. Is this a new problem that has just shown up with me? Or, has it been around a while and I just haven't been aware of it? Could it be poor quality repro parts? I do not know exactly what my engine rebuilder used. Any advise would be appreciated. Jim in San Jose.