Also, managers generally don't like their people going to HR about things without already knowing the situation ahead of time.
Yep. This part. Even if you convince HR, your manager will be able to come over the top and say no. And if they feel like you are an HR risk, that’s exactly what will happen.
Manager. Ideally, you want your manager ready to fight HR to keep you before reminding HR of what they told you. Also, managers generally don't like their people going to HR about things without already knowing the situation ahead of time. They don't want to get a call from HR and be surprised, even if its something innocuous.
save