Hi @ptrblck, thank you for your reply; indeed I was using track_running_stats=False. However, I am using dropouts with default settings (even if they are AlphaDropouts, to be precise).
If in validation, batch norm uses the wrong stats, that could explain the drop in performance.
However, I am more puzzled by why using a loader (num_workers=1) gives different results wrt to not use it; and, by why using multiple loading processes (num_workers>1) gives different values at each run.