So when I copied the file into the question above, I copied from the Mathematica front-end, which interprets the file contents slightly differently. The contents of the file is actually

    #!/usr/bin/env wolframscript
    
    M[a_, T_] := (4 π^2 a^3)/(G T^2) /. {
            G -> Quantity["GravitationalConstant"]
    }
    
    (* Assuming inclined around major axis, no change to major axis *)
    m1 = M[a, T] /. {
            T -> Quantity[15.7, "Years"],
            a -> 0.183 * 60^-2 * π/180 * Quantity[7.9, "Kiloparsecs"]
    } // UnitConvert;
    Print["Assuming inclined around major axis: M = " <> ToString[m1, InputForm]]
    m1S = UnitConvert[m1, "SolarMass"];
    Print["\t = " <> ToString[m1S, InputForm]]

I had input π as a unicode symbol. Wolframscript didn't interpret this as the constant Pi, but when I open the file with the front-end it automatically converts any instance of π to `\[Pi]`. The command line input was therefore unable to evaluate the expression as it doesn't know what the symbol π is, and there also seems to be some encoding issues in the output.

Strangely the unicode input seems to work if I invoke the file with `wolframscript -script`, but not just with `wolframscript` or with `wolframscript -file`.

Sorry to waste anyone's time with an incorrect question.

Mea culpa!