You said "the echo behaviour stops when the recording button is turned off".
NO MATTER WHAT RECORDER you are using the behaviour is exactly the same - the problem is in the ROM.
When I make or receive calls with my handset, the other caller hears their voice echo loudly but the recording of their voice is clear yet records my voice only very faintly. It almost sounds as if it is a speakerphone call.
When I make calls from my bluetooth headset, the caller does not hear their voice echo and the conversations is clear and normal, however in the recording the callers voice is not audible or not recorded.
I am using Note 10+ SM-975U1 unlocked (not rooted, sadly), Android 11 with accessibility enabled and microphone disabled for Google. I have Boldbeast Pro 14.9 GP set to auto record, default settings, record mode 12, noise suppress, Tune Audio Effect, Tune Audio Route 1, Tune Samsung1, Tune Samsung2
I have been using the Pro version for many, many years on my old rooted Note 4 and really hope we can find a workaround for my new device.
Thank you for any guidance you can provide.