Nested reference type record values and nullable constraints

Given the following examples I can't quite figure out what the compiler is doing here.

```fs
type InnerRecord = { Id: int }

[]
type StructRecord = { Value: obj }
// Ok
let nullable = Nullable({Value = 1 })

[]
type StructRecord2 = { Value: InnerRecord }
// A generic construct requires that the type 'StructRecord2' have a public default constructor
let nullable = Nullable({Value = { Id = 1 }})

[]
type StructType(value: obj) =
member _.Value = value
// Ok
let nullable = Nullable(StructType(1))

[]
type StructType2(value: InnerRecord) =
member _.Value = value
// A generic construct requires that the type 'StructType2' have a public default constructor
let nullable = Nullable(StructType2({ Id = 1 }))

[]
type StructType3(value: StructType) =
member _.Value = value
// Ok
let nullable = Nullable(StructType3(StructType()))

[]
type InnerStructRecord = { Id: int }

[]
type StructType4(value: InnerStructRecord) =
member _.Value = value
// Ok
let nullable = Nullable(StructType4({ Id = 1 }))
```

Why specifically is it not allowed to store a struct that has a reference type record as one of its fields? (And then secondly, if it is expected behavior the error is supremely unhelpful in figuring this out)

This issue seems related to a previous issue https://github.com/dotnet/fsharp/issues/7946#issuecomment-687266672 but the linked explanation reads like it should only apply to generic structs.