A wave of closures and cutbacks is sweeping through China’s private art museums, ringing alarm bells about the sector’s sustainability and raising questions about the outlook for one of the world’s biggest art markets.The Jupiter Museum of Art, in Shenzhen, announced its shutdown in June. Days later, Qingdao’s TAG Art Museum followed suit. Others, such as Ennova Art Museum in Langfang, founded by the company behind Hong Kong-listed ENN Energy Holdings, have been dormant for months.Even the...