Hi Philou
I'm on a my dev machine with zen 154, and I see now that inserting a product bookx is not adding the date_added in the products table.
I've check the online shop, and I have that field filled, but I can reach a conclusion because:
first - it's a zen 151 and the bookx module it's also different.
Second - I actually upload the files as general product and convert them later.

Can you confirm this, checking you database ?

Thanks
Best Regards