Hello,
I take a look at your test website and I was able to perform some tests.
For what I could see, your issue is not related to HIkaMarket since we can see the same display issue within an HikaShop order (not only in the HikaMarket sub-orders). Since the display is identical, what you can see in vendor orders is just a duplicate of what you can have in customer orders.
In the classical HikaShop invoice, we can see a problem with the display of item fields (where values are in a product field) ; only the last element was display fully.
It might be a problem in the HikaShop invoice view ; but I admit that it's possible that the front-end HikaMarket invoice do have the same issue (since the code for the invoice in HikaShop and HikaMarket are very similar).
Regarding the "PDF Invoice" plugin ; there is no display of item fields but the current code in the plugin should display it.
By enabling/disabling the plugin, I was able to compare the result between the HikaShop invoice and the generated PDF invoice.
So I don't know if you do have an old version, if you do have a customization or if it's a bug.
The HikaShop support team would be able to tell you more about it. But since I was not able to see "classical" item field in the PDF invoice, I do have the feeling that it's not related to the item+product system. You do have some orders with text item fields and they are not displayed in the PDF invoice either.
Because the problem is also in HikaShop orders, I do think that it will be better to open a new thread in the HikaShop section of the forum but HikaMarket do not interact with it. The patch should be first in HikaShop.
Regards,