Key signatures.

I'd bet you're using a transposing instrument, like a Horn in F, an instrument transposing by a 5th, in that case this is normal and by design. If you want one sharp, use the key sig with none, C-Major

MuseScore ask the key signature for the UT instruments, and transpose for the transposing instruments.
For exemple if you start with a concert band score, with many different key S. MS asks you the tonalité of your future piece. If you want in C, MS write the C key S. for the UT instruments, and automatticly, write in D for the Bb instr., and in A for the Eb , and in G for the F.
If you start with only 1 instrument like you in F like Horn, if you ask in C , MS write i G for your F instr. Try this, instead enter only a F instrument, enter also an UT inst. like flute, enter C for Key Sig., you will see MS correctly put in C for the flute and in G for the F instr.
If you have only a stave with a F instr. you must transpose in your mind :you want G for your F instr, you enterC (for the UT) and all is OK