Basile Bernard

Well, I do not aggree 🙂

First, you could add something like
if($date==null) ...
in the class.sitemaps.renderer.php file to prevent things like <lastmod>(insert incorrect timestamp here)</lastmod>

Second, I am pretty sure that I am not the only one to have had things like :
register_taxonomy_for_object_type( 'post_tag', 'demande' );

In my functions.php
There is a way for you to list all the post_types that are using taxonomy in order to get from this :

$args  = array(
					'post_type'      => $tax->object_type,
					'tax_query'      => array(
						array(
							'taxonomy' => $tax_name,
							'terms'    => $terms,
						),
					),
					'orderby'        => 'modified',
					'order'          => 'DESC',
					'posts_per_page' => 1,
				);

to this :

$args  = array(
					'post_type'      => [insert array of different post types here],
					'tax_query'      => array(
						array(
							'taxonomy' => $tax_name,
							'terms'    => $terms,
						),
					),
					'orderby'        => 'modified',
					'order'          => 'DESC',
					'posts_per_page' => 1,
				);

(class-taxonomy-sitemap-provider.php line 118 and after)

That would prevent the date error above (and solve my problem, for now I hacked your code with hiddious patch)

Thanks for your time