APs do not have the ability to trigger such type of messages in end-users device, I assume the end-user device runs certain connectivity tests to determine if it has internet, I suppose one or more of these tests might have failed and therefore they are seeing this message.
My recommendation is to have a baseline, first, you need to know exactly which connectivity tests these devices run to determine if they have internet, afterward run those tests and leverage the packet capture feature in the dashboard, monitor mode captures, anything that you can, check your device IP configuration and ensure it is as expected, ping your DNS servers and gateway from your device.
