Maybe the model is almost perfect already (?)
What is mLm probability, and is it supposed to have a capital L?
Is your data very similar to the data originally used to pre-train BERT?
What value of loss do you get if you try to fine-tune using the data originally used to pre-train BERT?
Maybe there are a few differences, but BERT has encountered enough examples of each difference to learn the patterns within the first 150 steps.