Try opening your WhatsApp and the locking it in memory and make sure you select never to offing wifi when the screen is off and lock.
As for the google gms cna check can use UOB mighty fx?
The phone can be L1 certified in terms of security and Google certified but xiaomi will still need to pay a very expensive fee to Netflix whitelist their phone, which I doubt they did it since this model meant to be in China only
Other apps works fine, banking apps and grab. Just notifications do not come through once screen is off. "opening WhatsApp and locking it in memory"? i have changed settings to no restrictions for battery saver.