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.
K Hatto
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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.