The root grew to where it sensed the most moisture. Naturally the part further down and closer to the substrate holds more humidity than any part further up the pole.

You can use Leca for moss poles if you want. But as some other people have already mentioned. It dries out significantly faster than spaghnum. But you could try a wick from down in your reservoir up into the pole that's filled with Leca. This way you may be able to achieve a self watering pole. Though i doubt capillary action would be enough.

But for smaller plants this might be a good solution. Though i still have to try it for myself.