No source records when Matsyendranath died. He is known only through Nath and Kaula textual tradition and through ongoing worship in the Kathmandu Valley as the rain-bringing deity Machindranath, celebrated annually rather than commemorated as deceased, which supplies no death tradition an ordinary biography could draw on.
What would resolve this An external documentary or epigraphic record independent of Nath, Kaula and Newar devotional tradition naming an end to Matsyendranath's activity.
OpenHistory of Hinduism, Nath and Newar religious studiesDavid Gordon White, The Alchemical Body: Siddha Traditions in Medieval India