[BUG][typescript-fetch] generator does not map Java Date object to string

Description

I'm unable to generate proper typescript code, because the generator does not convert swagger Date definitions into string, even though I've explictly used the type-mappings argument.

openapi-generator version

4.0.3

Command line used for generation

java -jar ${openApiGenCliJarDownloadDir} generate 
     -i http://myexample/v2/api-docs 
     -g typescript-fetch 
     -o src/api/myexample/generated-client-open-api 
     --type-mappings=Date=string  
     --additional-properties modelPropertyNaming=original 

Result

export interface MyInterface {
    /**
     * date of birth in YYYY-MM-DD format
     * @type {Date}
     * @memberof MyInterface
     */
    date_of_birth?: Date;
    /**
     * decease date in YYYY-MM-DD format
     * @type {Date}
     * @memberof MyInterface
     */
    decease_date?: Date;
    /**
}

Start by reproducing the reported Java CLI command with the typescript-fetch generator and inspect the generator's handling of Date definitions and type mappings. Done means the generated interface uses string rather than Date for the reported date fields when --type-mappings=Date=string is supplied.