Why the validation accuracy is constant for all the pre-trained models while applying on medical datasets with .czi format?
weights: One of None (random initialization), 'imagenet' (pre-training on ImageNet), or the path to the weights file to be loaded. Default to imagenet.

With weights=None, it is randomly initialized and
With base_model.trainable = False, it is not being trained.
So, basically your Conv. layer is not getting trained.