What device was the person with the missing voice using?
Recording on iOS is less reliable than on a computer or Android and can drop out at the start or halfway through a session. When that happens we message the user and ask them to reload to reconnect the recording but they might not always to do so. If the user with the missing voice was on an iOS device (iPad or iPhone), then that is the most likely explanation.
If they were on another device then please let us know at firstname.lastname@example.org so we can investigate further.