JustDummies

Both mechanisms report a replay instruction, and nesting makes the outer one false.

Any.Reproducibly takes its seed from Guid.NewGuid().GetHashCode() — not from the ambient source — so an inner scope ignores whatever the outer one pinned and draws afresh on every run. The outer runner, or [Reproducible], still reports its seed. The failure therefore says “reproduce this run with seed N”, and replaying with N reproduces nothing.

That is a wrong instruction rather than a wrong result, which is what makes it worth reporting: the reader trusts it and loses time before doubting it.

Noncompliant

[Fact, Reproducible]
public void It_is_accepted() {
    Any.Reproducibly(() => { /* ... */ });   // JD018: the attribute's seed governs nothing in here
}

Compliant

[Fact, Reproducible]
public void It_is_accepted() {
    /* ... */                                 // the attribute already pins the whole test
}

What it does not flag

• The seeded overload — Any.Reproducibly(1234, ...). Pinning a chosen seed inside is deliberate, and the reader who wrote it knows which seed governs what.
• A lone runner in a test that carries no [Reproducible].