When Radioisotope Dates for Recent Volcanic Eruptions Date Anciently What Does That Say About Unwitnessed Events?

Said to be good rock samples to radioisotope date from the lava flows of the 1980 Mount St. Helen’s eruption turned out to be bad samples when they dated in the hundreds of thousands or millions of years of age by various methods, so what are we to think about the radioisotope dates of other lava flows worldwide not witnessed by human eyes?