First of all, thanks for the great ROMS. Miui is awesome :)
I am hitting the same bug in miui jb 2.8.24 as in ics 2.8.24 (see below).
Enabling the screen lock after proper wipe and re-install leads to being locked out.
http://miuiandroid.com/community/threads/pin-code-screen-lock.17806/...