Let's face it this is happening to many people including me and used to work in the past. This has nothing to do with our sitemaps. It is a bug that somebody should solve somewhere. I am not worried because the blog is spidered  and posts appear in the search results anyway. I have seen too many possible solutions here to something that is not to be solved by users. Sometimes resubmitting the sitemaps the thing gets solved and then they disappear again. I guess this will be fixed someday somehow so let's not worry too much as long as search results are the same. It is evident that a know issue means a known issue and it should come either from a bug in your system or whatever change being implemented. Anyway thanks for your kind support.