Sitemap duplicate files issue

For some reason yesterday my sitemap was changed. Instead of having 19 forums files with 10,000 threads each, they changed into showing only 1-12 files and then duplicating 1-7 again. Meaning files 13-19 are gone. Should I generate a new sitemap in hopes that this corrects itself?

Yes, try to delete all xxl.gz files from the ftp and re-run it manually to see what happens.