I have confirmed that a similar bug can be reproduced on iPad OS 15.1. The method is to turn off the iPad for half a day and then connect to the device via Wifi again. Null will be returned after 20 seconds. After another 4 seconds, the SSID will be returned again. This only happens once after the iPad is turned on.

I had the same problem, so I submitted this to the Feedback Assistant. OpenGL for iOS is implemented on top of Metal. This issue is a bug in AppleMetalGLRenderer that provides an OpenGL implementation. To avoid this, use MTKView instead of GLKView.