Does the research disappear completely, or does it just disappear from the discounted techs on the left of the screen? Is it included in the list on the right side of the screen, but you aren't looking there?
There is bug I've reported multiple times, with saves, since April.
Sometimes when you complete a trade, your current research is cancelled. You then have to chose what to research, but the tech you were researching is still available. If it was one of the discounted techs on the left of the screen, it probably won't be in the discount list after being cancelled. It will be in the list on the right side of the screen. You can choose it there, but it will be at full cost instead of the discount cost.
If the tech isn't available in either list, you have found a different bug. Whatever the case, a save should be useful for finding the problem, if anyone will actually look at it.
