That’s just the default behaviour for when you have more than 1 million values - essentially the histogram turns into a density at those very fine resolutions.
You can pass fill = true if you just want to fill the area under the curve, not sure how you get the actual bars to show but imho it doesn’t look great when the bars are hardly wider than their outlines - this is what it looks like for 1m observations, where bars are still plotted: