There is, of course, one way to test. To see if the fabric is to blame or the iron is to blame. Take a freshly laundered pillow slip or sheet or tea towel or shirt. Plain white for preference, but any plain light or pastel shade will do.

Iron it as you did your kilt. Use lots of steam - extra bursts and just normal. Marks or no marks?

Marks or grit and it is your iron or water.
No marks, then it is the kilt.

Let us know.

Regards

Chas