Mail deletes all Inbox and Sent messages even if not selected for deletion

This one is odd, and at first I thought I was imagining it, however having lost a couple of very important emails I know it's definitely happening.

If in Mail and you want to delete messages from Sent or Inbox, you can either do it individually, select a bunch, or select all. The problem I'm facing is that if I select all of them then deselect the ones I want to keep, the phone deletes them all anyway!

Sometimes it will make it look like the unselected messages are there, but if you then go back in, either instantly or after some time, they will have been moved to Trash.

The problem there is I've been emptying the Trash folder not realising it had contained important sent emails which are now gone forever, so every time I now want to empty the Trash I have to go in and move the emails back to the relevant folder even though they should have been moved.

If I deleted messages individually, it's fine. I'm not sure what happens if I manually select a bunch either individually or select one and hold my fingers down to do the quick-select method.

Anybody else experiencing this or got any ideas?

Thanks in advance

Mitch