In my shop some of the products have an attribute-option to select the electrical impedance, which is listed in ohms with the symbol Omega Ω

When the invoice prints, the option is indicated not with the Ω symbol, but with the code Ω ; (ignore the space before the semicolon, I needed to do that so the code would show up instead of they symbol on the forum).

What have I done wrong or what is causing this and how can I fix it so on the invoice the Ω symbol shows as Ω?