Gmail deleting emails automatically

[Admin]

I've had that occasionally with Gmail.  Sometimes the missing email has been swallowed up into a "conversation", sometimes it just vanishes.

The "conversation" function is really annoying -  I used to turn it off but lately there don't seem to be a way of doing this.

Yeah I had wondered about that myself, but seems to not make any difference, so far its doing it with emails received from another gmail address.
I have no idea where the emails are going, but they appear to be deleting automatically without being marked or labelled as trash or spam.