Video call picture in picture not working

When I am in a video call (snapchat and WhatsApp). If I return to my home screen the chat doesn't go to picture in picture mode. If i got straight to another app through a notification, or swipe down and go to my settings app, the call then goes to into picture in picture, I can then go to niagra home and it stays.

Current home behavior:

• I'm on a call

• I swipe from the bottom to go home

• My call closes

Current work around

• I'm on a call

• I open my notifications and open and app through a notification, or go to my phone settings

• My call goes into picture in picture

• I can then swipe to niagra home and the call is visible in a window.