I've flown from the Uganda to the US many times and have never seen this. However there is a current "outbreak" of Ebola going on there now. Only a few dozen cases out near the Congo border from what I've seen, but people freak out about Ebola.

You are not required to be proactive and can certainly pretend ignorance. But if you are asked by an immigration officer where you have been, you had better be honest because they can see your travel history.

Also be prepared for some sort of screening or testing at the next airport you arrive in from your Entebbe flight. I know Nairobi has put in place screening of pax, but it is usually just a temperature check.