I had this problem myself last night. Went to use split screen to multitask and it got stuck there and the regular method did not work for me. Just found this online and it worked perfect. So it appears there is a change in the method.
If your screen is black or frozen
If your screen is black or frozen, you might need to force restart your device. A force restart won't erase the content on your device. You can force restart your device even if the screen is black or the buttons aren't responding. Follow these steps:
On an iPad Pro 11-inch or iPad Pro 12.9-inch: Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Then press and hold the Power button until the device restarts.