The implementation at

// Optional fields for tuple representation are only allowed at the end, and contiguously.

However, the current spec at https://ipld.io/docs/schemas/features/representation-strategies/#struct-tuple-representation says:

Optional or implicit fields are not possible with the tuple Struct representation strategy, all elements must be present.

For now, bindnode follows codegen; see #368.

We should probably adjust the docs to agree with the codegen. cc @warpfork @rvagg