Hello, it seems that you are right. I tested it today at work and it eventually locked, but I think about an hour later, in spite of auto lock being set to 10 minutes.
Now I tested it at my home mac and now I'm at 12 minutes and so far it did not lock, again, it is set to 10 minutes.
I assume dropbox is accessing it for a moment and this prevents the lock, but nevertheless I will let the dev team know, maybe they will have some further input.
Cheers
Zsolt