Comment # 11 on bug 1101393 from Stanislav Brabec

I can confirm that. I suppose that xscreensaver either locks on resume (or it
locks on expired timeout), or the lock operation on suspend takes too long and
suspend process does not wait for finishing.

There are several other problems with xscreensaver:

If any application blocks suspend, and root password is required, xscreensaver
locks before displaying the prompt, not after entering the password.

Regarding the fade out, I would prefer keeping that, as it is more comfortable
when you are reading. But even there is a problem: If locking is configured, it
happens on fade out start, not fade out end. So if you are reading, and move
the mouse to keep screen displayed, you have to enter the password. Locking
when fade out (triggered by timeout) ends would be more comfortable.

And sometimes (I still don't know when and why), xscreensaver-managed session
is locked by xlock (a different screen locker). I did not identified the
trigger yet.