Did you work at Sikorsky in the 1980s? It seems to have been the preferred spelling there, to the point of my being admonished for using the correct spelling after showing them the entry in the dictionary
Information about American English usage here and here. Floating point issues? Please read this before posting. — emc
These words like 'pneumonia' and 'mnemonic' came from greek. And not only in english they came - that's the key point. When these greek words with diphthong 'eu' have adopted in russian language it receives a strong 'v' sound in place of greek 'u'. So, the 'pneumonia' sounds like 'pnevmonia' in russian, but 'mnemonic' is still 'mnemonika'.
I've done some research on google and yes, it's μνημονικά and πνευμονία.