Just a guess: your bike probably didn't flex, but you unweighted the front wheel when standing, which affected the steering -- made it light and twitchy. It's normal for your weight distribution.


You can try remembering to lean forward a bit when getting out of the saddle to keep a larger fraction of your body weight on the front. You might try adding weight to the bar bag or adding front panniers to keep the front wheel weighted. Either of those risks making the steering sluggish. You might think when you stand and unweight the front, the added weight will make the steering like an unloaded bike, but Murphy's law says that doesn't usually work.


MHO, just go with the flow and enjoy the ride. Other options include joining a supported tour, where the tour operator carries all your luggage. It's a different kind of touring, with less flexibility w.r.t. schedule and stops, and less opportunity to meet the people who live there. So in the end, you've got to pick one approach for your next tour and deal with the consequences.