I've just came across something fairly puzzling.
I have several restricted surveys where some lines in the token table are flagged as "completed" but some of the corresponding entries are missing in the answers table. And the crazy thing is that respondents whose answers are missing did received the confirmation mail.
So, it seems that their answers either disappeared or were not recorded at all.
I've looked in both web server and sql app logs and I'm still without a clue about what's going on.