try without setting trainer.training=True

src/lightning/pytorch/loops/fit_loop.py

@@ -253,10 +253,7 @@ def setup_data(self) -> None:
 
         combined_loader.limits = limits
 
-        trainer.training = True
         self._data_fetcher = _select_data_fetcher(trainer)
-        trainer.training = False
-
         self._data_fetcher.setup(combined_loader)
         iter(self._data_fetcher)  # creates the iterator inside the fetcher
         max_batches = sized_len(combined_loader)