This is weird.
I was using Windows 7 Pro SP1 and a February driver.
Then I fresh installed Windows 10, upgraded to whatever latest version is (all updates installed), and installed the 18.5.1 driver.
After rebooting, I noticed one of the two fans on my Sapphire R9 270X stopped working properly. For instance, they spin normally on POST, but then Fan A stops spinning. It sometimes comes back to life on the desktop, but the weird thing is it stops spinning after I open a game.
If I set their speed (via graphical UI) to e.g. 100%, then both fans spin fast, but this Fan A is definitely spinning on a lower RPM than Fan B, not too much though.
I took the card off the motherboard to test the wire and connectors (the ones that connect to the PCB). Both spin at the same speed, so the fans/cable/connector are NOT the problem.
Has anyone ever seen this? GPU-Z is reporting a GPU BIOS version from 2014, so I don't think the BIOS was updated while installing the driver.
Performance in games is unaffected by all this. I'm now playing GTA V. While the GPU temp never reached 70+ºC, this temp was achieved after a few minutes of gameplay. I'll continue testing.