"OpenStreetMap maps world as it exists, and includes mapping borders and countries according to actual current situation and not a preferred or ideal situation." (https://wiki.openstreetmap.org/wiki/Disputed_territories)

OSM follows the de-facto policy which is equal to every country. China also has many historical material to support its claim from of old until in now, like 诸蕃志 said China has administrated this area since A.D.789 (under 振州's administration), 四海测验 (1279), 琼台外记 (Ming), 琼州府志 (1841), etc. If every country maps as its claim, the map would be in a mess. So, OSM follows the de-facto to map the real world rather than somebody's imagination world.