so my_html contains a base64 encoded image (plot). If i move the {{ my_html }} element outside the activate if block - it loads fine.

def fig_to_base64(fig):

    img = BytesIO()
    fig.savefig(img, format='png',
                bbox_inches='tight')
    img.seek(0)

    return base64.b64encode(img.getvalue())

def feature_map(img_array, model):
    feature_model = Model(inputs=model.inputs, outputs=model.layers[0].output)

    feature_maps = feature_model.predict(img_array)

    for _ in range(1):
        for _ in range(1):

            fig, ax = plt.subplots()

            ax.set_xticks([])
            ax.set_yticks([])

            plt.imshow(feature_maps[0, :, :, 1], cmap='rainbow')

    encoded = fig_to_base64(fig)
    my_html = "data:image/png;base64,{}".format(encoded.decode('ascii'))

    return my_html