Bug:
If you have a trade offer in your notifications (not opened). Save the game then load that save. The trade offer option goes away. Loading the auto save of that turn seems to give the trade offer still as long as other notifications are in front of it. However you don't always have an auto save. it is of particular importance due to the fact that I save before I check a deal in order to be able to view my techs list to see if its a good deal or if I want it. As currently your unable to do so from the trade menu.

Possible Cause:
Im guessing the trade offers disappear due to the fact that the global number made by the RNG changes every time you save a game. Therefore loading it removes the trade offer as that number is changed on save/load.

Solution:
Make trade offers independent of the RNG, or at least if one is generated have it save what the offer was so when you load a game it is still there.

Notes:
This issue is also apparent if there is a notification from the AI such as a Demand for tech/credits, a plea for help, a declaration of war/peace. or pretty much any interaction with the AI.

This can be used to cheese the AI if for example they declare war on you, all you have to do is save an load before the notification, or one turn before and in most cases it skips it for awhile.