IMAP Deleted Messages disappearing

I have several IMAP accounts and they are set to never remove deleted messages. However, whenever I run my mail program on the iPad and view the Trash folder, it "cleans up" and I lose all Deleted Messages older than a week old. Now, I know I should probably not delete messages until I am 100% sure I am never going to need them, but I simply don't work like that and, in theory, the iPad should not be doing this.

Has anyone any ideas? It's as if the ipad is ignoring its own settings. Or could this be an IMAP problem? I have looked on the server setting for my email provider and there is nothing there - plus if I don't use my iPad for a while then the messages don't disappear from my other devices.