Hi,
this looks like a building and was mapped as a building, but you have removed the tag building=yes. Therefore it is no longer rendered as a building.
If it is a building, it should have a building tag, no matter what other tags it has.
The tag leisure=horse_riding is not really for individual buildings; it should be used for the entire area of riding centres, compare:
https://wiki.openstreetmap.org/wiki/Tag:leisure%3Dhorse_riding
If the building is a riding hall / indoor arena, you can use the tag
https://wiki.openstreetmap.org/wiki/Tag%3Abuilding%3Driding_hall