First the IRS (feds) don't care where you live as they tax all income earned worldwide.
Next ... if you live and work in TN then why would you not inform your employer so that you can stop paying PA state and local taxes ? Check with the state to see if they will still tax teleworkers even if you don't live in PA any longer. https://ttlc.intuit.com/turbotax-support/en-us/help-article/state-taxes/states-reciprocal-agreements...
My employer has a residency requirement that I must live in PA, but allows me to work remotely. I want to take advantage of the remote working and be closer to family. So if I don’t tell my employer of my move, and work in another state, I wondered if that would cause issues for taxes
Q. How will my taxes work each year?
A. You federal tax doesn't change. You use the address you want the IRS to communicate with you at. For state taxes, the normal rule, is that you would no longer be subject to state tax at you employer's location. But, Pennsylvania does things differently.
If you work outside the state as a job requirement, you are only subject to Pennsylvania State income tax on the days you work in PA. But if you work outside PA for your own convenience (as in your case), you are subject to PA State income tax on all your income. New York, Nebraska, Delaware and Arkansas have the same rule.