Hi,
you did not add an equestrian area but you retagged a building to an equestrian centre (leisure=horse_riding) and this is wrong because
1. you removed the building=* tag and the building is no longer rendered as a building
2. the equestrian centre is larger than the building, the area should cover the entire site of the equestrian centre, like here: https://www.openstreetmap.org/way/1039425581 (or map a node for it).