Does flutter disappear above the flutter speed?
Flutter happens when elastic and aerodynamic frequencies converge. Normally, the elastic one is (almost) fixed and the aerodynamic one increases with flight speed. When speed increases further and the frequencies move apart again, flutter will die down. However, with further increasing speed more elastic modes will be encountered with ever smaller gaps between them.
There are indeed cases of airplanes which have a flutter mode below minimum speed but are safe to operate within their certified speed limits. The linked article doesn't mention this, but if you fly that thing slowly enough, the tail surfaces will wildly swing caused by a torsional elastic mode of the rear fuselage being in frequency neighborhood with oscillating separations on the tail surfaces. Pretty scary but harmless at that low speed.