Second and other users not available on the BPM index

Hello, I cannot add a second or additional user to the Garmin Index BPM. I have installed the device properly and the device takes over my user name immediately. I then add my wife as the second user and our son as the third user. The invitations are also received by my wife and our son and they can also confirm the invitation. The Index BPM then also appears in the app under ‘Devices’. But now comes the problem: the new people are not synchronised to the Index BPM. I have tried several times with my own measurements and manual synchronisation. Only my name and the anonymous ‘Guest mode’ continue to appear on the device. I have already uninstalled the device several times and also reset it to the factory settings. For test purposes, I have also installed it on the phones and under the accounts of my wife and our son as ‘owner’ and then set myself up as a second user. But the problem always remains the same. I can install the first user (owner), but another user is not synchronised to the device. Does anyone have any idea what the problem could be and know a solution?