The beginning of the English word "fuchsia" is not pronounced like the German word Fuchs, so indeed the spelling does not match the pronunciation. This is independent of the fact that it comes from that word. Plenty of things in English (and, in fact, loanwords in every language) sound different from the words they're derived from; that doesn't mean trying to imitate the source language is the "right" pronunciation. If you pronounce fuchsia like "fuksia" nobody will understand you.