So what is the problem, actually?

Paper does not just vanish. It takes one to five years to break down. In the mountains it is even slower, because low temperatures and dry air hold it back.
On top of that, tissues contain wet-strength agents that keep them from falling apart when wet – handy while you use them, less so out in nature.

Left behind, they

• are a grim sight for whoever comes next;
• can be eaten by animals;
• release chemicals straight into nature.

Alternatives?
Take your tissues back with you, use none at all – or, best of all, switch to a reusable PeeCloth!