Mannes

Mannes is actually a short form of Hermannes. In turn, Hermannes is a variant of the more common Hermannus... which is a variant of Hermanus that remains faithful to the Dutch pronunciation of Herman. You see, the pronunciation of the -a- changes with the addition of -us, but not with -nus (which contains an extra consonant). It's a quirk of the Dutch language that I'm not sure how to explain, because it's very intuitive and natural for native speakers. Hopefully this makes it a bit clearer:

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Herman = /'ɦɛr.mɑn/
Hermanus = /ɦɛr.'ma:.nʏs/
Hermannus = /ɦɛr.'mɑ.nʏs/

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Also compare the Dutch word 'man' meaning "man" and its plural 'mannen' meaning "men":

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man = /'mɑn/
mannen = /'mɑ.nən/

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If you were to write 'manen' instead, then the pronunciation would be /'ma:.nən/. The meaning would be different as well: it means "moons" in Dutch.

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In short: two of the same consonant keeps a vowel short, while only one consonant makes a vowel long (when a word consists of more than one syllable).

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Sources:
• Hermannes: https://nvb.meertens.knaw.nl/naam/is/Hermannes (in Dutch; shows the prevalence of the name in the Netherlands in 2017)
• Hermannus: https://nvb.meertens.knaw.nl/naam/is/Hermannus (in Dutch; shows the prevalence of the name in the Netherlands in 2017) [noted -ed]

Mannes is also Jewish, a form of the Hebrew name Manasseh. It is seen in the name of the famous American public figure Bernard Mannes Baruch (1870-1965).