Preprocessing fields that are used as rewrite tokens

I have some code in template_preprocess_views_view_field() that modifies the output of a field.

That field is excluded from display and then printed via token in the "Rewrite the output of this field" option.

So for example, I might have a view that shows node title and username.
The node title field is excluded.
The username field is rewritten to print "[title] by [name]".
I have custom code in template_preprocess_views_view_field() that modified the markup for the title field.

The end result is that my markup changes in the the preprocessor are ignored when using the rewrite tokens.

Is this a bug, or is it by design?