This is a bug that only affects the Post Title block at the moment, but would affect more as soon as the block selectors specificity is higher than the user-selected styles, as per #29155 in which we're opening up the block selectors to target inner blocks.

Step-by-step reproduction instructions

• Use a theme with support for FSE (templates) and Global Style (theme.json), such as TT1-blocks.
• Via theme.json target the post title for color styles by adding:

"core/post-title/h2": {
    "color": {
        "text": "green",
        "background": "yellow"
    }
}

• Go to the editor, add a post title select some text and background preset colors for it (not custom).
• Publish the post and go to the front end.

The expected behavior would be that the user choices are respected. Instead, the values of theme.json are applied.