HTC One M8 :: Email Randomly Disappearing From Stock Mail Client?

I have recently purchased myself and another of my colleagues an HTC One M8. We also have another colleague who has had an M8 for some time.

The two new devices I setup as ExchangeActiveSync accounts on the stock mail client. The other user has set his up as IMAP/SMTP. As a company we use office365 for our email solution.

Here is an example of the issues I am having:

This morning I received 11 emails, they all came into the inbox fine, I then received a 12th email. At this point the previous 11 emails were removed from the inbox. However all these email still remained on the server. It doesn't seem to have any pattern, it could be the first or the 20th email I receive and it doesn't always remove all the previous emails. It could just be the last 4 or 5. The Sync period was set to 30 days.

The other user who has the IMAP setting says he experiences the same issue and always has done.

I have tried other mail clients on the phone and they seemed to work, I also have the same account on a Google Nexus which works without issue.