I got 2 problem in 2.8 cycles render

Hello! I was excited to try 2.8 stable 3 days ago but still have some problems what would need to solve, both connected to rendering. I have an AMD 3900X and a Radeon VII and when I activate both checkmarks in preferences I thought it will be a CPU+GPU render but instead it use only the CPU all the time, even if I set GPU computing, the GPU just works at idle hz-s and CPU is working. I would want hibrid renders for fastest results possible. If I turn off the CPU checkmark my GPU renders fine

UNTIL

I try to render the Spring scene from the blender cloud I would want to try benchmark tile sizes on that but when I try to render with my GPU the windows freeze and restarts. I stress tested my GPU with furmark and works fine, under games works fine also, under my previous 2.79 projects I just render with 2.8 with GPU works fine, but the Spring scene just crash my system. Why? I would want to have stable system and fast GPU and GPU+CPU renders all the time. How we could solve this? How can I colelct logs and send to developers maybe?