So it seems that even though you have a quite large class imbalance, your model still overfits to the minority class without any loss weighing?
This seems yo be quite an interesting use case, as you would usually see the opposite.
Could you try to decrease the pos_weight even further and see if the model starts to predict the majority class at one point?
If that doesn’t help, could you post the code you are using to get the predictions and calculate the confusion matrix, as you might have a bug there.