Changes do not show up through revision diff

Which "XML sitemap fields" are you referring to exactly? Can you provide a screenshot?

Changes we make to the XML sitemap in the vertical tabs, we would like these changes to show up in revision diff. Attached screenshot.

I have no knowledge of the internal workings of the diff module, but sitemap entity overrides are not stored as entity fields. Instead they are stored in a custom table "simple_sitemap_entity_overrides". I assume what you are asking will work when (if) #3034070: Store entity instance settings as fields on entity → gets implemented. Let me know if this needs more clarification.

This makes sense, I assume it will likely work when https://www.drupal.org/project/simple_sitemap/issues/3034070 → is implemented, hopefully at some point this upgrade is made.

This isn't really fixed though, it should probably be closed (won't fix) or closed (duplicate).

This is a support request that's been solved and not a feature or bug ticket. Even if it were one, it wouldn't be a duplicate or won't fix.

Automatically closed - issue fixed for 2 weeks with no activity.