In my macbook ⟦⟧ are fallback to a system font by default, so maybe you should install an other font that support this symbol, and goldendict should be automatically fallback to this font. just a guess
Probably your computer system does not contain certain font in which such symbols are included,my Windows 11 displays them corectly, I suggest you change your system to newer one because better systems work better, we need not know which font really support the symbols