I’m afraid I don’t have much to offer on this one. I don’t play loud, but in my experience some notes tend to go flat over time and a need for re-tuning a note is generally because it’s flat. I don’t understand why or how this happens. In your case, I guess you know why but not how. I would tune the reed and see whether that fixes the problem or whether it happens again. On one of my P32Ds, the middle G suddenly went seriously flat. I tuned the reed up, and it proceeded to go flat again the next day. I decided there must be a hairline fracture in the reed. I hope that’s not your problem. It’s interesting that the same thing happened to two identical melodicas under the same conditions. If you add three more P-25Fs, you’ll have a scientific experiment — sorry, I know this is no joking matter.