Booting Manjaro or Arch on a Lenovo E595 with an AMD Radeon RX Vega 10 GPU on the latest amdgpu drivers gives a beautiful corrupted display:
The only solution I found for recent kernel/firmware/drivers combinations is to set
amd_iommu=off
as a kernel flag. See this blog post for the implications.
Fortunately, the latest drivers do not seem to suffer anymore from infamous Waiting for fences timed out crashes (see this issue) that happen on earlier linux-firmware versions.