Usage including that in the respective Wikipedia pages shows "Martenot" in "ondes Martenot" and "theremin" are capitalised differently.
Léon Theremin developed the theremin in 1918, and Maurice Martenot invented the ondes Martenot in 1928. "Ondes" is “waves” in French.
Why does one keep the capitalisation of its creator's name but not the other?