The swing arm pivot and front sprocket if not actually on the same axis are very close to it so there is very little tension change with suspension travel.
If the belt is going slack within a short distance, the culprit will be inadequate tightening of the wheel spindle. The adjusters are simply that; they adjust the position and alignment of the wheel spindle, the only thing that stops the wheel spindle wandering off from its set position is the friction created when the spindle is tightened. If you set the alignment and belt tension perfectly but don't tighten the spindle enough the adjusters are quite happy to deform and allow the belt to slacken and the alignment to wander off. Cue the belt jumping teeth and the wheel bearings taking a pounding.
Good luck with the chain conversion but please keep it aligned, tensioned and well oiled, there isn't much to stop a snapped chain from visiting the inside of the cockpit.
