Account with UID 501 disappeared after update

After today performing a system update from 13.10 [previously updated cca 1mo ago], my account with UID has disappeared from lightdm.

/etc/login.defs, /etc/adduser.conf and /etc/lightdm/users.conf have all been configured to treat users with minimum UID 500 as regular users.

The remark in users.conf mentioning AccountsService gave me a hint to explore what this service is. It's very nasty that this completely opaque and unknown service determines whether an account will appear on the list or not.

Digging around its process /usr/lib/accountsservice/accounts-daemon (what a nasty place to put this binary) using strace, I noticed /var/lib/AccountsService/users/myusername is being accessed. This may be obvious to developers, but to someone troubleshooting, it's bad and unexpected.

I fixed my issue by editing the aforementioned file and replacing "SystemAccount=true" with "SystemAccount=false", killing the accounts-daemon and (to avoid restart) starting it in terminal. This would all be worth just a rant instead of a bug report, were it not for the fact that only today have I logged in correctly from the main screen, updated the system, and after only a reboot -- without touching or knowing about /var/lib/AccountsService/users directory -- my account has disappeared. Either this directory was automagically touched, or "SystemAccount=true" previously didn't affect lightdm.

Please provide and expose a canonical and universal place to determine minimum UID for the system to treat a user account as a "human" account. Having three somewhat-documented files and none of them having effect upon accountsservice is quite bad.

If this package is inappropriate for this bug report, please reassign.

[The reason why UID 1000 is no-good are OS X partitions -- on which I'd like to be able to access my files without resorting to sudo voodoo. The default UID on OS X happens to be 501.]

Package version: 0.6.34-0ubuntu6

ProblemType: Bug
DistroRelease: Ubuntu 13.10
Package: lightdm 1.8.5-0ubuntu1
ProcVersionSignature: Ubuntu 3.11.0-18.32-generic 3.11.10.4
Uname: Linux 3.11.0-18-generic x86_64
NonfreeKernelModules: wl nvidia
ApportVersion: 2.12.5-0ubuntu2.2
Architecture: amd64
Date: Tue Apr 1 00:43:23 2014
InstallationDate: Installed on 2014-02-18 (41 days ago)
InstallationMedia: Ubuntu 13.10 "Saucy Salamander" - Release amd64 (20131016.1)
MarkForUpload: True
SourcePackage: lightdm
UpgradeStatus: No upgrade log present (probably fresh install)