Volume on 3 way GROUP Facetime call VERY LOW to one device only - ios 17.1.1

This is a hard and inexplicable Facetime audio problem to define, so please bear with me.

In this scenario, there are 3 iPads all running the latest ios build (17.1.1). I shall refer to them as A, B and C

iPad A is actually an iPad 2019 (7th Gen), 128 GB running ios 17.1.1

iPads B and C are both identical and recent purchases - iPad 2021 (9th Gen), 64GB, running ios 17.1.1

When we are set up in a 3 way Facetime group call:

a) A can hear B and C clearly

b) B can hear A and C clearly

c) C can hear B clearly but can barely hear A

However, with the problem active, if A switches the call from iPad 2019 to his iPhone 13 device (128GB, running ios 17.1.1), the audio springs back into high volume for the user on iPad C. All the time, the user on iPad B is unaware of any problem.

This has really got me scratching my head because it appears to make no sense at all !

And yes..we have tried, rebooting, resetting, checking for a muted mic, checking for a low volume setting. All looks normal. And in any event, if a setting was wrong on iPad A or C, why is it Facetime calls work fine (audio wise) between A and B, B and C, and even A, B and C if A used his iphone instead of his iPad ?