I just received this response from Amazon:
Greetings from KDP support team.
Given your query, yes, you will still receive the royalty amount based on the list price you've selected and not the promotional price reflected on Amazon. A customer might purchase your book at a discounted price that’s lower than your list price due to promotions, but it won’t impact your royalties.
Per our terms and conditions, we retain discretion to set the retail customer price for the books we sell. Our decision to discount products is based on a number of considerations which vary over time. Royalties you earn from books printed to fulfill orders through Amazon.com, Amazon.co.uk, Amazon.de, Amazon.fr, Amazon.es and Amazon.it will be based on the list prices you set for your book, not the retail customer price.
So I will be paid for books sold whilst the price was discounted.
But since the book went back to its normal price after I emailed them, perhaps I should have left well alone and the discount might have stayed in place!
Still, worth knowing in case of anyone having their paperback discounted in future that you will be paid the usual royalty.