Could you post a screenshot? Were these orders placed by Business Customers?

In such case you would need to provide them an invoice or a receipt according to the policy:

https://sellercentral.amazon.co.uk/gp/help/help.html/?itemID=GHTQ5U9J25GBF5YB&ref_=ag_GHTQ5U9J25GBF5YB_h_r0_cont_sgsearch

Same with me, not VAT registered, appearing on every order, opened a case and received the following response:-

“Hello from Amazon Selling Partner Support,
From your email, I understand that you would like to know why amazon suddenly requesting you to send Invoices to Buyers.
Sorry for the previous inconvenience caused in this regard.
I see that in the past, if the buyer requests for Invoice then amazon was automatically providing invoices to them. So you want to know why amazon is now requesting you to send Invoices to Buyers and also implement a option for that.
Please be informed that previously amazon was providing invoices to all the order whether the sellers may be a Non-VCS seller or VCS seller, However, As per our recent update, Automatic invoice sending option was cut down to only VCS enabled seller’s and Non-VCS seller’s now have to provide invoice manually from their end.
In a wide survey, Amazon as found that the seller’s are use buyer seller messaging interface to send invoice to the buyer and using this feature most of the customers are not receiving the invoice.
This is was the main reason why this option was enabled in Manage order’s page. So to improve the experience, Amazon has launch this functionality to allow sellers to upload invoices where they see a new “Upload Invoice” button on their orders page, which will also allow the invoice to be attached directly into the order page.
Once it is done, The buyer will be able to download the Invoice in their Order’s page without any issue.
Note: Kindly be informed that uploading Invoice for all the order as became mandatory whether it is a business order’s or not. So please do upload invoice’s for all the Order’s so that the buyer will be able to download it from their end.
I believe the above information helps.
If you have any further questions in this regard, please don’t hesitate to write back to us. We will be glad to assist you.​
Please let us know how we did.”

In a word, dreadful, it is now compulsory to upload invoices for all orders if you don’t have a system in place to do it automatically. What a joke.