Nullness issue - constructors for DU cases consumed in C# do not show nullability

[T-Gro]

Issue description

When consuming a reference DU from C#, the constructors of the generated subclasses do not respect nullability of the fields.

Choose one or more from the following categories of impact

• Unexpected nullness warning (false positive in nullness checking, code uses --checknulls and langversion:preview).
• Missing nullness warning in a case which can produce nulls (false negative, code uses --checknulls and langversion:preview).
• Breaking change related to older null constructs in code not using the checknulls switch.
• Breaking change related to generic code and explicit type constraints (null, not null).
• Type inference issue (i.e. code worked without type annotations before, and applying the --checknulls enforces type annotations).
• C#/F# interop issue related to nullness metadata.
• Other (none of the categories above apply).

Operating System

Windows (Default)

What .NET runtime/SDK kind are you seeing the issue on

.NET SDK (.NET Core, .NET 5+)

.NET Runtime/SDK version

No response

Reproducible code snippet and actual behavior

[<NoComparison;NoEquality>]
type MyDu = 
    | JustLabel
    | JustInt of int
    | MaybeString of nullableString:(string | null)

Possible workarounds

No response