@shikijs/rehype: transformers in postprocess hook is not run

[olets]

Validations

• Follow our Code of Conduct
• Read the Contributing Guide.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.

Describe the bug

When processing code with shikijs/rehype, a transformer in the postprocess hook is not running… I think. I'm new to remark, and might be missing something in my parse / rehype / plugin / stringify setup.

Willing to work on a PR, but am new to remark, rehype, and this package.

Reproduction

https://stackblitz.com/edit/shikijs-rehype-transformer-hook-shikijs-shiki-884

Contributes

• I am willing to submit a PR to fix this issue
• I am willing to submit a PR with failing tests