Why are the Antonov An-124 horizontal stabilisers directly behind the wings?
Flutter is the main reason why the horizontal tail on Antonov designs is mounted low. Many other transport airplanes do indeed sport a T-tail (C-141, Il-76, C-5, C-17, A400M).
Cargo airplanes with a rear ramp suffer from low torsional stiffness of the rear fuselage because of that large cut-out for the ramp doors. If the ramp doors are locked with many locking points around their circumference, this can be remedied but requires those locks to work well and reliably. The Antonov philosophy is to play it safe and assume that Russian maintenance crews will clear the plane even if not all locks engage.
A T-tail adds a significant mass on the lever arm of the vertical tail and lowers the eigenfrequency of the rear fuselage. This brings the torsional frequency of the tail in the range of the first antimetric flutter mode of the wing, as happened on the C-141. Putting the horizontal tail low avoids this and allows for a lighter structure.
Since the wake of the wing moves down with the downwash angle, it passes below the tail of the An-124 in cruise. Not by much, but enough to keep flow over the tail clean. At higher angles of attack the wake will hit the tail, but those should not be encountered in normal operation. If that happens, the main effect is a lower dynamic pressure on the tail, not flutter. For that to happen, you need some coupling of aerodynamic and structural oscillations. For slow flight speed, the wing flaps will bring the wake again well below the horizontal tail.