We are working on solving a problem that we're currently seeing with IMAP indexing on our mail servers. The symptoms customers are likely to see are small oddities such as emails appearing not to move between folders, or appearing twice. This problem is only index related and so doesn't actually affect the emails themselves. This problem is not causing email to be lost.
We are still investigating a proper fix for this problem, but in the mean time we are making changes that should work around it. There is a small risk that if you are using Sieve filtering that it may stop working. If that is the case, please contact support for assistance
Longer term fixes for this was implemented as part of planned work: https://aastatus.net/2487