Nodes wrongly marked as not accessible by anonymous user in xmlsitemap table and not included in sitemap.xml

I have a drupal 7.52 website, I noticed that the number of entries in the sitemap.xml file constantly reduces with time even if the number of nodes on the websites is the same or increases. Rebuilding the sitemap "resets" the problem and all nodes are included in the newly generated sitemap.xml. But again every day the number slowly reduces. I checked the xmlsitemap table in the database and I found out that many nodes which are accessible by anonymous user have 0 in the access column. When I rebuild the sitemap all nodes have 1 in the access column, then after cron is run a few times, some nodes which are accessible by anonymous user have 0 in the access column, every time cron is run some more nodes are wrongly marked as not accessible by anonymous users and thus removed from the sitemap.xml file.
Any hint on where to start to debug this problem? Thank you.