Make sure the GPU fans spins freely without any resistance.
Also make sure you have all the PSU PCIe GPU Power cables connected. One cable per GPU Power connector if applicable.
Those GPU card should have Zero RPM feature where the fans start working when a certain temperature is reached. normally anywhere from the mid 50c to 60c.
If under heavy loads and the GPU card temperature reaches 90c without the fans turning on does seem to indicate there is certainly something not working correctly.
Does the GPU Fans start running for about a second or two when you power up? If it does then you know the fans do work since BIOS checked the GPU card and the Fans started working for a second or two which is normal.
You can always install the GPU card in another computer to see if the fans work or not.