I have read the FAQs and checked for similar issues: Yup

My site's URL (web address) is: Message ID: 593f1ceb2d67

If I go to /webmasters/tools/sitemap-list?hl=en // one minute the sitemap index is included in my overal count (all content submitted will show 250k).
Moment's later it's gone and the count only shows around 1k (the value of a few other sitemaps (not indexes) i included for reference)
Then, It'll come back, then it'll go, it's random.. at first I paniced and resubmitted everything.

Now I'm guessing you have a bug?

Cheers,
Frank.