Whilst @m-Elghamry didn't actually solve my problem, he did force me to relook at the issue and it turns out there was a separate field that also needed to be initialized that was actually causing the issue. The compiler was just sending me on a wild goose chase after the wrong property.

Essentially the issue was that the record required 11 constructor arguments and the mapping only catered for 9 of them. So I had to use the [MapValue(...)] attribute on the missing fields and map then to a function call to supply the appropriate value, case closed.