According to Wikipedia, the lake is both called "Lilla Stamsjön" and "Långa Stamsjön". The main name is given as "Lilla Stamsjön" on Wikipedia, so the OSM data seems correct.

https://sv.wikipedia.org/wiki/Lilla_Stamsj%C3%B6n_(Lerums_socken,_V%C3%A4sterg%C3%B6tland)