imessage group receiving recordings of unrecorded phone calls made by members of the group

I am the organizer of a four person imessage group. Everyone uses one or more of iphone, macbook, or ipad. Three of the participants are located in Toronto. The fourth is located in Austria.

Earlier this week the Austrian participant had a private phone conversation with a party in the USA unrelated to the imessage group. The Austrian participant did not record the phone call. Later in the day (Sept 4) a partial recording of of that phone conversation appeared on the imessage group and was accessible by all participants. That participant further reported that later in the day, the recording of their conversation disappeared and it was replaced by a partial recording of sound in the home of one of the Toronto participants.

Today, on the device of the Austrian participant the "Toronto recording" disappeared and was replaced by the recording of the earlier phone conversation.

How can we prevent this from happening?