Order Status Changing Back
If you change an order status for an imported eBay order, then the status reverts back to a previous status - it is because you have used a status that is unknown to the eBay module. When the next update happens for that order it will change back to the previous known status.
You can only use a status that is listed under:
OpenBay Pro > eBay > Settings > Settings tab > DEFAULT IMPORT SETTINGS
When you change an order status in OpenCart, you must only use the ones you have chosen in that list.