Ringing tone still heard after call has been answered

I’m experiencing a rather strange problem. For some calls, not all of them, the ringing tone can be heard in the background together with the conversation after the call has been answered. It happens for any call flow: internal to internal, internal to external and external to internal, so I can rule out my initial assumption that the SIP trunk could be the culprit.
The strangest thing is that, unless I’m missing something on the SIP debug, there are no noticeable problems with the call session. Since it doesn’t happen for every call, it is quite difficult to debug, but anyway I caught quite a few of those calls but found nothing wrong with the SIP session. I googled this a lot, but found nothing but some guy saying this might be due to the call not being bridged correctly, with no further explanations. Could I be hitting some sort of bug? I’m using FreePBX Distro Stable-2.210.62-5 with Asterisk

This is the pattern related to who will hear the ringing tone even after the call has been answered:
internal to interal: called party
internal to external: called party
external to internal: calling party