Last Date Modified Not Update in Sitemap. Below URL for example.
Last modified date is 14-12-2020 but in sitemap its shows 23-12-2019.
https://meta.discourse.org/t/configuring-facebook-login-for-discourse/13394
<time itemprop='dateModified' datetime='2020-12-14T21:13:21Z' class='post-time'>
Above URL available in sitemap 3.