It's not so unusual for prose writers to write poetry too. They used to publish them in separate books. The situation's less clear nowadays. For example, consider "Certain Windows" by Dan Burt (Lintott Press, 2011). It's 64 pages long, with poems from PN Review and the TLS. There's a 35-page prose autobiography of his early years. How did such a situation arise?
Many decades ago, there were places in the US/UK that published snippets of prose - I think Readers Digest had little pieces for example. But these outlets dried up so the authors of these short texts, if they wanted them published, had to send them to poetry magazines. Of course, prose-poetry existed, but that term was reserved for surreal, discontinuous works. So what they did was add line-breaks. Also popular was the idea of making all the stanzas the same size of rectangle, as if there was a metrical/rhyming pattern. Read Paul Durcan's poems to see how it's done.
Then Flash emerged, providing a natural home for short narratives again. Various other short prose formats became popular too. Authors of short pieces no longer needed to add gratuitous line-breaks. Some authors have taken advantage of this. Carolyn Forché has re-published her famous "The Colonel" poem as prose. Don Paterson publishes books of aphorisms
I think it's time that editors questioned line-breaks that do little, just as they challenge words and lines that don't pull their weight. Some editors do, but line-breaks have so many putative purposes (many so subtle that I can't see them) that editors tend to leave the layout alone.
Just to add to the fun, there's a trend to use "/" instead of a line-break. Some people use multiple spaces between words. In addition more people than ever are writing poetry with an aim to be published, so it's no surprise that people are spreading into less traditional areas of poetry, straying into hybrid zones.
The conflation of short prose with poetry has led to more books having a mix of poetry and prose. E.g. -
- Helen Tookey's "City of Departures" (Carcanet, 2021) begins with poems, but ends with pages that are unashamedly prose.
- "Citizen" by Claudia Rankine won poetry prizes but much of it wasn't trying to be poetry, it seems to me.
- "Small Hours" by Lachlan Mackinnon (Faber, 2010) ends with a long section of text which I'd call prose. Some reviewers said so too.
Nowadays poetry readers seem capable of not caring about line-breaks. When they start reading a poem I think they decide whether it's the sort of piece where line-breaks matter and read the piece accordingly. Neither do they care much if there's obvious prose in a poetry book. I suspect it's been going on covertly for a while. I read a U.A. Fanthorpe book recently. It looked like a mixture of poetry and prose. Her famous "Not my Best Side" is like the prose I try to write. I doubt if the Trades Description Act can be applied. That said, I think Poetry judges could be braver.
If you can't beat them, join them. I have prose and poetry versions of some pieces. I’ve short-lined and long-lined versions of poems. I've even (shame on me) taken a paragraph from a story of mine, added some line-breaks, and had it published in a poetry mag.