@jp.stacey Agree with you on:
Markup: It should not be an array of strings. ...

@jp.stacey Agree with you on:

1. Markup: It should not be an array of strings. When we do that then there are fatal errors with various warnings.
2. Placeholders: Yes the placeholder values should be renderable arrays which can contain not only markups but other Renderable elements as well

I tried what is mentioned in the documentation for the placeholder as well as what @jp.stacey has given as an example, Where the later worked without any errors.