JAKARTA, Indonesia—A year ago, clerics here in the world’s largest Muslim-majority country expressed alarm over China’s treatment of ethnic-minority Muslims—around a million of whom have been detained in re-education camps, according to human-rights groups.

Leaders of Muhammadiyah, Indonesia’s second-largest Muslim organization, issued an open letter in December 2018 noting reports of violence against the “weak and innocent” community of Uighurs, who are mostly Muslims, and appealing to Beijing to explain.

Soon...