Sitemapp missing entries

Hi,
since yesterday (21. June) the number of entries in my media sitemap
(https://hi-flyfoto.de/attachment-sitemap.xml) suddenly dropped from 155 to 4, without any changes by myself. Deactivating and reactivating the sitemap make no difference. Yoast is the current version, 3.3.2.
Has anyone else experienced this?
Also, I have read in the Yoast support that it might not be a good idea to sitemap images. Is this true?
Thank you for your help
Michael

https://www.ads-software.com/plugins/wordpress-seo/