Thanks to a tip from Paul Puri.
According to multiple topics on the Apple Support discussion boards, the problem can occur when a user logs into their Mac's Guest account — whether by accident or on purpose — and then tries to log back into their regular account.
In some cases, users have reported finding their regular account empty of data, as though it were a brand new account.
Unexpected problems do sometimes prop up on new versions of operating systems. I am sure this will be fixed in the next revision. Dealing with unexpected glitches is the price of early adoption.