Rust and Beam crash on explicit constructors with a base and then #5271

An explicit constructor that has a base call and a then block crashes on Rust and Beam. .NET prints 20 1 1.

let mutable thenCalls = 0

type NBase =
    val Id: int
    new (id: int) = { Id = id * 10 }

type NChild =
    inherit NBase
    val Extra: int
    new (id: int) = { inherit NBase(id); Extra = 1 } then thenCalls <- thenCalls + 1

let nc = NChild(2)
printfn "%d %d %d" nc.Id nc.Extra thenCalls

• Rust: Fable crashes with The lists had different lengths, raised from makeRecord in Fable2Rust.fs.
• Beam: it compiles, then crashes at runtime with badmap in fable_utils:field_get.

Removing the then block fixes both, including with fields written out of declaration order.

Reproduced with ./build.sh quicktest rust and ./build.sh quicktest beam on 7e12c10. #5235 doesn't change these results.

On Rust, the same crash also hides the Fable error that #5235 adds for new (...) as self = { inherit NBase(id); ... } then .... The build still fails, but with the crash instead of that message.

I haven't looked into the Beam side. For Rust, a review comment on #5235 points at the constructor handling in Fable2Rust.fs, which only inserts the base field for Sequential(baseCall, NewRecord(...)). I haven't confirmed that's the cause.