assign variables gets command not found

helper.sh

function check_tag_exists() {
    local tag=$1
    local return_var=$2

    log_func_call ">" "${FUNCNAME[0]}" "${FUNCNAME[1]}"

    # Inicializamos la variable de retorno.
    local tag_found=1
    local found=1

    # Establece una trampa que se ejecuta cuando la función sale.
    #trap 'log_func_call "<" "${FUNCNAME[0]}" "${FUNCNAME[1]}"' RETURN

    git ls-remote --tags origin | while read -r line; do
        echo "Tag ${tag} en linea ${line}." >&1
        if echo "$line" | grep -q "refs/tags/${tag}$"; then
            echo ">>>>>>> Tag ${tag} found." >&1
            #tag_found=0
            #eval $tag_found=0 fails
            #eval "${tag_found}"=0 fails ./templates/scripts/release_helpers.sh: line 135: 1=0: command not found
            found=0
            echo ">>>>>>> tag_found ${tag_found}" >&1
            break
        fi

        echo ">>>>>>> 1 tag_found ${tag_found}" >&1
    done

    echo "******tag_found ${tag_found} ****" >&1
    if [ "$tag_found" -eq "0" ]; then
        echo "******Tag ${tag} found****" >&1
        eval $return_var=0
    else
        echo "ELSE ******Tag ${tag} not found****" >&2
        eval $return_var=1
    fi

    eval $return_var=$found
    # Establece una trampa que se ejecuta cuando la función sale.
    trap 'log_func_call "< res $return_var" "${FUNCNAME[0]}" "${FUNCNAME[1]}"' RETURN
}

ALWAYS get value 1, why ? not works in while-do ?

        check_tag_exists "$releaseVersion" tag_exists
        echo "tag_exists ${tag_exists} value."
        if [ "$tag_exists" -eq 0 ]; then
          echo "La etiqueta ${releaseVersion} existe."
        else
          echo "La etiqueta ${releaseVersion} no existe."
        fi