Answered in the comments:
"Why can't there be two or more flowerbeds right next to each other, upon which a single person lies? There is nothing wrong with either sentence." – Jason Bassford
Also - flowerbeds often seem to have sections of different flowers. They can be varied and extensive throughout a park (although they don't necessarily have to be). Using the plural subtly connotes this information - it brings to the reader's mind that maybe there are many flowerbeds, with many sections.
The English language itself is not as strict as physics equations. There's room for poetic interpretation. She might be lying in only one flowerbed, but surrounded by a few others.