Security: what to do when you tell Google Family Link to stop monitoring a device, but continue to monitor the device.

I was using the parental control software "Google Family Link" to monitor a device. I wanted to stop monitoring the device. Therefore, in the Family Link parent application, I pressed "Stop monitoring".

Google sent an email saying that it had stopped monitoring. But Family Link continued to show a notification on the device that said it was under supervision, and still continued to prohibit certain actions on the device.

Even restarting the device did not help.

What is the solution?