Because the option to continue playing after being interrupted by other applications is turned off, just turn it on. Taking the iPhone 6 as an example, the method is as follows:

1. Open QQ Music and select in the upper right corner options as shown below.

2. After entering, find the setting option and click to open, as shown below.

3. Find below that the option to continue playing after being interrupted by other applications has been turned off, just turn it on.

4. Just open the options on the right, as shown below.