I have no idea. So i just force disconnect if the RSSI of the WiFi is too low.

Is the M5Stack WIFI scan area larger than smartphone? When my smartphone cannot scan my wifi, M5stack still print that it can scan and connect to the wifi.

technically they should have the same range however metal in the case and crappy antenna design on the phone can cause issues.
Also the phones software may decide not to work.
If a phone is not switched off (fully off not standby or sleep) data stored in memory will cause the phone not to detect properly.
I have seen this with samsung, sony and apple phone, and also with both linux, windows and OSX based computers!
Best tip I can find is to delete all stored connections to wifi networks, switch off wifi, Do a full powerdown of pc/phone. wait 5 mins, then reboot device.

(If you use M5ez, you can see if it's still connected, and it will auto-connect when it sees a know access point, so that's easy to test with...)

Thank you.

I also want to ask that will it auto disconnect from the wifi when it is no internet access? Or I have to use WiFi.disconnect() to force it disconnect? when i walk too far from the router, it still connect to the wifi but cannot connect to internet

I've also noticed it's not bad at all for a device whose makers could spend only a fraction of the development budget of the Apples and Samsungs of the world. But then smartphones have way, way more throughput than an ESP32.