Svelte reactivity affected by initial variable value

I've started working with Svelte and I stumbled upon some unexpected behavior. This is the snippet of my app source

    let currentPage = 1;            $: if (currentPage || $selectedOption) {                fetchPage(currentPage, $selectedOption);            }            async function fetchPage(page: number, option: string) {                //...            }            onMount(() => {                fetchPage(currentPage, $selectedOption);            });

When I load the page for the first time fetchPage() function is called twice. However, if I'll make this change let currentPage = 0; it is properly called only once. The problem is that I need to have currentPage value initially set to 1.

I've tried also this change:

let currentChange = 0;// ... rest the sameonMount(() => {        fetchPage(currentPage, $selectedOption);        currentPage = 1;    });

But this didn't help as well. Similarly any bool flags set in onMount() and then checked in if statement didn't help.

Any suggestion what else can I do?