I know this is a year old, but for others who like myself found your question to be the first search result the answer is in my case and most likely yours is that Inertia.js preserves state for all requess other than GET as per their documentation here: https://inertiajs.com/manual-visits#state-preservation it took me a good hour of searching around before I stumbled upon that section of the docs.
For example:
form.submit(
'post',
'https://example.com',
{
preserveState: false,
}
);