Loader remember it's first stack and don't accept any other items.
Steps to reproduce:

1. Place into loader some coal.
2. Wait until it loaded to wagon.
3. Place into loader some iron ore. It will not loaded.

Now loader accept only coal. And I don't find any way to reset this except loader destruction.

Filter updates when a train arrives. Put coal and ore into the loader first, and both will be loaded when the train arrives.

Very strange mechanics.

Loaders/inserters are usually two different types: filtered and unfiltered.
Maybe you better provide two different loaders too? With a standard filter settings interface?
Or at least make this behavior optional?

Very strange mechanics.

Oh...
It seems that I realized why you had to do this. And how it works. Sorry.