***** NOT A TEACHER *****
(1) I have found an expert who says there is a difference, BUT he himself says that neither sentence "is more normal than the other." In other words, the difference makes no difference!
(2) IF (repeat: If) I understand this expert, here is the unimportant difference:
(a) Mona: No one knew for certain what was the matter with her.
George: I knew. Losing her youthful looks was the matter with her.
Mona: Poor baby!
(b) Mona: No one knew for certain what the matter was with her.
George: I knew. The matter with her was losing her youthful looks.
Mona: Poor baby!
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I found this information in a famous book entitled A Dictionary of Modern English Usage by the famous H.W. Fowler. He wrote it in the ancient days of 1926. Today many people ridicule (make fun of) him as being old-fashioned. Other people feel, however, that he will be the master until the end of time. (I have the second edition, 1965, page 279.)
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P.S. If I had my druthers (that's how we old people used to say "If I had my choice"), I should choose the second sentence.