[deleted] After your reply, I actually went and created 2 new separate profiles and setup the system settings to my preference for both of them (to check if it is an app which is messing up something). And after that, I noticed that I was never asked for pin and FPR always worked when switched in between these new profiles, but when switched from either of these new profile to one of my older profiles (created and setup last year), I had to enter pin to unlock.
Today, I formatted my spare device (P7a with GOS) and set it up with only 2 profiles with my system settings of choice on both 'owner' and 'New Profile'. I thought it should not ask me for pin and FPR should work every time I switch in-between these 2, but it is not the case. I am having similar exp as of my main device (P8 with GOS) - I had to turn of the screen after each profile switch and then have to enter the pin to unlock it. Later on the same device, I created a 3rd profile and set it up. Now, when I switch from 'New Profile 1' to 'New Profile 2', FPR always works. It is just the case that whenever I try to switch to/from 'owner' profile, it fails.