Speleothem 618O records from central southern China have long been regarded as a key benchmark for Asian summer monsoon intensity. However, the similar 618O minima observed among precession minima and their link to seasonal precipitation mixing remains unclear. Here, we present a 400,000-y record of summer precipitation 618O from loess microcodium, which captures distinct precession cycles similar to those seen in speleothem 618O records, particularly during glacial periods. Notably, our microcodium 618O record reveals very low-618O values during precession minima at peak interglacials, a feature absent in speleothem 618O records from central southern China. This discrepancy suggests that the mixed summer and nonsummer climatic signals substantially influence the speleothem 618O records from central southern China. Proxy-model comparisons indicate that the lack of very low-618O values in speleothem 618O records is due to an attenuated summer signal contribution, resulting from a lower summer-to-annual precipitation ratio in southern China at strong monsoon intervals. Our findings offer a potential explanation for the long-standing puzzle of the absence of 100-and 41-kyr cycles in speleothem 618O records and underscore the critical role of seasonality in interpreting paleoclimatic proxies in central southern China. These insights also have broader implications for interpreting speleothem 618O records globally, advocating for a more multiseason interpretive framework.