That does not sound good at all !
Some have said that after a short while of using the phone the warming issue goes away but some have also said they are still experiencing problems.
Usually it's owners of the Exynos version.
My suggestion is to either seek support from a Samsung Experience Store or Samsung Service Centre or if your within any relevant cooling off period have the phone swapped out.
I wish you all the best with this situation.
Current Phone >Samsung Z Fold³ 5G.
Back Up Phone > Note 20 Ultra 5G SM-N9860.
Was it just a return for refund and repurchase or was it dealt with under warranty?
Which department did you return to and were sent a new one from?
Tech support/Warranty said they can only repair and can't replace to me.