Margaret Mitchell never published anything besides Gone with the Wind. There was a manuscript discovered after she died which was published. Maybe something else of Lee’s will surface later.
From Wikipedia...
For decades it was thought that Mitchell had only ever written one complete novel. (In fact, periodically claims are made that she never wrote it at all due to the lack of any other published work by her). But in the 1990s, a manuscript by Mitchell of a novel entitled Lost Laysen was discovered among a collection of letters Mitchell had given in the early 1920s to a suitor named Henry Love Angel. The manuscript had been written in two notebooks in 1916. In the 1990s, Angel’s son discovered the manuscript and sent it to the Road to Tara Museum, which authenticated the work. A special edition of Lost Laysen — a romance set in the South Pacific — was edited by Debra Freer, augmented with an account of Mitchell and Angel’s romance including a number of her letters to him, and published by the Scribner imprint of Simon & Schuster in 1996.
Umm, she was also a newspaper columnist who had a couple hundred articles published. Plus, that manuscript you mentioned and lets not forget she died young only 10 years after her big novel. ..
If this is the the template explaining the paucity of writing from 84 year old Harper Lee (who wont even talk about her only novel); its not a perfect one by any measure.
‘Lost Laysen — a romance set in the South Pacific’
A not uncommon mistake for a new writer — writing about something exotic and which you have no empathy for, then writing a good book which you feel internally