Validation/calibration of clinical prediction model using km estimated survival

first time poster - primarily a clinician (medical oncologist), but pursuing a part-time PhD to develop teaching materials for clinical prediction models (CPM) for clinicians.
I am looking to review a commonly used oncology predictive tool, and in the process of reviewing the published validation/calibration papers, I am finding a reasonable number are not using true observed endpoint (death), but an endpoint estimated using kaplan-meier. I cannot find a discussion of this in the evolving literature around reporting, validating, and calibrating CPMs that addresses this approach, and while it raises a non-specific concern for me about the impact of censoring and potential introduction of bias, I am not knowledgeable enough how to delineate this concern explicitly, nor how concerned I should be. Any thoughts on this would be greatly appreciated.