Facetime audio does not get recorded

Hi all

Please help.

I am having Facetime calls with my mom and I want to record them as memories. But when I use the in-built iOS 11 recorder (from Control Centre) on my iPhone 7+ (iOS 11.2.6), the audio does not get recorded.

I already know how to turn microphone on (by doing the 3D touch thing), but still the audio does not get recorded for Facetime calls.

If I record screen + audio on my own (without Facetime, just random clicks on the screen with a voiceover), it works. But when I am doing a Facetime call and turn the recorder on (along with microphone turned on), no audio is recorded.

Is there some issue with my Facetime? Or my phone? Or has Apple disabled audio recording for Facetime application? If Apple has disabled, it should spell it out clearly so that people like don't end up wasting time looking for an in-built solution.

Tks.