Operating system: Ubuntu 20.04
Graphics card: GTX 980, GTX 970, GTX 660TI
Short description of error
Disabling Hair BVH makes Cycles CUDA GPU hair ribbons flicker between frames
Exact steps for others to reproduce the error
Disabling Hair BVH in 2.9 when using Cycles on GPU and ribbon hair makes hair ribbons flicker in animations, even when there is no movement. See attached file for a demonstration (rendering just a few frames should be sufficient).
Enabling hair BVH makes the problem go away, as does enabling 3D Curve hair primitives.
This is possibly either an issue with older GPUs or OS. The 980, 970, and 660TI all have the issue. The 1070 does not. However, the 1070 is running on my Windows desktop and the other cards are running on Ubuntu desktops. I haven't had time yet to boot the Windows desktop into Ubuntu via live CD and test.
This is probably related to https://developer.blender.org/T78152 but may contain additional helpful information.