Aspirin seems to have worked for you for some time, so probably it is less likely that you have aspirin resistance, as defined by studies. Probably something has changed in your illness and aspirin itself is not sufficient to reduce headaches anymore, for example more inflammation or more reactive platelets that clump faster into microclots. Microclotting and bleeding can coexist.
Another possibility might be that there is something else that is contributing to your headaches together with the MPN.
For me, aspirin alone could never solve the headaches I get from my hereditary thrombocytosis. So I have other headaches management techniques, including some mild physiotherapy exercises and ibuprofen patches that I apply on the back of my neck from time to time, so that I do not end up with headaches that last a whole week.

It would be good if you can discuss with your MPN specialist and with your neurologist again about these issues, so that you find a solution for your current situation. Chronic illnesses change over time, even if largely symptoms can remain unchanged, some symptoms can intensify. Hope they will help you find new ways to tackle the headaches.