scrollEndThreshold needs to cover the bottom padding too. The end-following logic only keeps following while the viewport is considered “at the end”; with paddingEnd: 120 and scrollEndThreshold: 80, sitting at the bottom of the messages but above the padded area can fall outside that window, so later dynamic height changes stop being followed.

Use either:

paddingEnd: 120,
scrollEndThreshold: 120, // or a bit larger

or reduce paddingEnd below the threshold. For async image/markdown growth, also make sure each row is measured with ref={virtualizer.measureElement} / data-index, otherwise the anchor cannot adjust to the real final height.