Would that transit visa still apply? Especially when visiting another country in between?

Can I travel trough Beijing (from Singapore), via Dandong to Sinuiju (North Korea, 1 day) and back? I'm flying from Singapore to Beijing to Dandong, will visit Sinuiju in North Korea and then continue flying from Dandong via Beijing to China. Total time between entering China in Beijing and leaving it to Frankfurt is less than 72 hours.