To better advise you would you please do an experiment for me.

Hold up your kilt by the two top buckles. You need to watch the area of the Fell or pleats between the buckles in the back of the kilt. Then pull on the buckles. You are trying to pull them apart just as they are when you strap the kilt on.. Do this firmly with about as much force as you would expect to happen while you wear your kilt, move around in it and bend. Say 20 pounds of pull.

Do you see any stretch or movement in the area directly between the buckles? Do the buckles or the tabs holding them to the kilt move at all? Is the stress of the pulling taken up by the stitching in the Fell at all? Does it look like the pleats are being pulled apart exposing the stitches.

If you see any stretching between the buckles the problem may be that the kilt maker forgot to put the stabilizer into the kilt. With the stabilizer in and well stitched down there should be no stretch between the buckles. None, nada, zilch. All the stress should be taken up by the stabilizer. There should be no stress trasferred to the stitching of the pleats.
Thank you for that, Steve. I did this experiment last night, and it is as you described. When I hold it by the buckles and pull, there is stretching between them. And I do see the stitching being pulled on.

What's weird is that when I lay the kilt open and look around the edges of the lining, I can see the very edge of what appears to be a stabilizer underneath. The lining is black, but there is a layer of firm tan-coloured material below it, which I would assume is the stabilizer. But if so, it is apparently not doing the job. I was pretty disappointed to see that.

So I guess here's where I eat some crow and say that my kilt is not as well made as I previously stated. The question is, then, what can be done about it? How difficult is it for a kiltmaker to take the lining off, evaluate what's below it, and (possibly) stitch a stabilizer in place?