You can try clean card from dust and replace thermal paste.Maybe this help.
I would not apply new thermal paste if your card is still under warranty. I am sure someone in this thread will have some better ideas.
The graphics card are newly purchased. I am not sure the undervolt setting is good! I need the undervolt setting to be set during system boot...
Some silly question, but maybe they can be useful.
How is the rig built? Open-frame?
How about the distance between GPUs?
What's the temperature in the room?
All those factors can massively impact GPUs temperatures, your software setting seem right