Passcode required to enable Face ID after any phone call

Over the past week, any time I begin/receive a phone call, even if the call doesn't connect, I'm required to enter my passcode in order to enable Face ID. Even if I hang up immediately after pressing the green phone button to make a call, it triggers the passcode requirement. This also happens after not using the phone for several minutes, but much less than four hours.

The only new behaviour in my engagement with the phone is that I recently installed a few new apps from the App store. I read that it's possible to get malicious code on the phone that way, so I've since deleted any recently downloaded apps.

Still the problem remains.

1) Is there a way to check if I have malicious code in the phone (and hopefully remove it)?

2) If I were to back up the phone now, after having removed potentially dodgy software, then restore it, would the malicious code remain in the backup?

3) Is there some other known cause for this?

-I'm not wearing a mask or glasses or anything else that alters my appearance.

-I've cleaned the screen / TrueDepth-sensor camera area very well.

-No dirt, cracks or deformations in the screen anywhere.

-No SOS, medical ID or Find My iPhone instances recently (for over a year at least).

-I've deleted and reconfigured my Face ID in Settings.

-Wallet, bank, insurance and other Face ID-enabled apps all behave as expected.

-The behaviour can be reproduced consistently by attempting to make, or receiving, a phone call, and sporadically by leaving the phone untouched for several minutes.

Any help is appreciated.

That said, if this is Apple's way of forcing an update, then I am officially ditching Apple for good.

TIA

iPhone XR, iOS 14.4.2