Hi Nicolas,
First, you are right, the free order validation plugin is activated. Not by me, I wasn't aware of this plugin, but maybe it was added after an update in the past. This is something I experienced several times before. I've started using Hikashop many years ago (2011) and I'm very happy with it and all the efforts you and your team are making for adding new features and solving problems to make it better every time.
But sometimes it;s difficult for users to see trough all the changes that are made in a latest version. And which options/fields are affected by these changes. Several times "normal" behavior was affected by these changes and we only noticed these after experiencing (some) problems on the frontend/backend. I understand it's impossible for you to forsee all the possible impacts on every situation, so this is not for you to blame, just explaining how this could happen. I disabled the plugin,, so problem solved I suppose.
I agree that "no sense" is just in my case, but I still can't see why anyone would have an invoice for a product of € 0,00 as an ordernumber would give the same information.