If you were using the CCC CPU Power setting previously, that had adjustable maximum and minimum limits, and would not lock the frquency. As for OverDrive in Radeon Software, it's very possible that an Over/Under-Clock will lock the CPU at that exact frequency. I'm not sure if it would be the same on your system, but I change my CPU speed from Windows' Advanced Power Plan Settings; which doesn't lock the frequency. It might be a better option than OverDrive for that purpose.
cpu running hotter is simply vcore is higher now , and personally get a better cooler then underclock a cpu is a far better way to resolve the issue
You have some good replies. Your CPU Core is based on the socket/chip and workload, CPU AUX or just CPU is the true temp. CPU Core will go below room temperature on idle (when your clock idles that is) and that's not possible. But if your clock is locked and no cores can sleep, or clock down, your CPU core will tell you its higher as its working. Why are you underclocking to start with, what's the cooler? If its the stock, and your not happy with the temps, disabling a couple or cores may be better.
If your still having problems post a screenshot of your CPU temps and clock speed on idle.