Mail Not Updating Other Devices

I have a filter on the work email that forwards all emails onto a folder in my Mail (which is a Gmail account through Exchange)

This folder has run up to a fair few thousand emails over the last few months so I've just been through manually on my iPhone and moved the majority of them to Trash. However on my iPad they still show as in the original folder. Same when I open the Gmail app. On the iPhone they are still showing as been moved to the new destinations.

There isn't usually much of a delay and updates are carried out instantaneously across my iOS devices.....any ideas?