I can pass something like this fine:
{this.props.post.auth
Try passing it as a React Element altogether, not as a string:
<CardTitle
title={this.props.post.title}
subtitle={
<span>
This is a link: <Link to={this.props.post.author}>{this.props.post.author}</Link>
</span>
}
/>
Then should be able to render subtitle
as is. If you're using React >16, you might want to use Fragments for this:
import { Fragment } from 'react';
// ...
subtitle={
<Fragment>
This is a link: <Link to={this.props.post.author}>{this.props.post.author}</Link>
</Fragment>
}