The strength of your rigging and the weight of the surface are what allows flutter to happen. Everything has a flutter speed, even a bowling ball. So if you get any model going fast enough, and your servos aren't strong enough or there is slop in the rigging, you'll see it. Well, you'll actually hear it but if it does what it usually does, and blows a wing or tail apart, THEN you'll see it.

You don't usually see models with mass balanced surfaces for a couple of reasons. But when you have a finished model, that's one thing that'll work. But it's almost the last thing to do.

Look at your rigging. Is the pushrod strong and supported so it won't bow? Does it connect to the servo arm and the surface horn without slop. Try to move the surface with your hand. Do that with the power on. If the surface moves easily, find out if the pushrod is bending or the servo is giving up. Flutter is the air pushing on the surface and winning the pushing match.

Sealing the hingeline sometimes works from stiffening up the surface. But what's already been mentioned is almost always the problem.

BTW, 3D models usually have surface sizes that are designed to work at very slow airspeeds. And the leverages found with their rigging gives the air a huge advantage over their servos. That's why a bunch of those designs have aerodynamic balancing as part of the plan. And you really won't make them flutterproof every time. FWIW........