MS have fixed it and are starting to reissue it. Back-up My Docs/etc anyway.
The problem came because a lot of beta testers move My Docs to a D: drive so when they reinstall, as they do a lot so as not to have to move data around. Windows, when running Clean Disk by default, did not include My Docs, but 1809 did, which the testers missed. Most of them. The new Clean Disk saw My Docs on D: and left it, but deleted it on C: including any data, which normal users still use, so it was us ordinary folk that got hit.
MS knew about this from months ago but even most of their secondary beta testers use D: so it didn't flag as anything other than an isolated case. What they don't have are normal user beta testers who leave everything as default. They'd have spotted it fast enough.