Is It Usual for There to Be an Unpleasant Odor Behind Temporary Veneers Prior to Fiting the Permanent Ones?

I don't know if "usual" is the right word, but it CAN happen.  Temporary veneers are designed to come off, so they don't seal/bond well to the teeth.  This CAN allow food/debris/plaque to collect and it can smell.  During the final bonding process to place the veneers, the area will be cleaned and disinfected, so it does not mean the case is compromised.  Rinse with hydrogen peroxide 1-2 times daily.  That should help.

The odor is not uncommon, usually due to "microleakage" of bacteria under the temporary veneers or crowns.  It is seen most frequently when patients are not careful with chewing on the temporaries, and that loosen the temporaries slightly, leaving a very slight opening for mouth bacteria to take up temporary residence in the tiny space between the tooth and the temporaries. 

When your dentist removes your temporaries he can remove the odor and any residual black discoloration in seconds by washing the tooth off with ordinary hydrogen peroxide.  The bacterial microleakage does not cause any damage.

If you have to be in temporaries for weeks, then you might return to your dentist.  He/she might try washing the edges of your temporaries with hydrogen peroxide, or else removing the temporaries, cleaning off the bacterial microleakage with hydrogen peroxide and then recementing the temporary crowns or veneers.