From your description, I think the Horizon Client from Google play store was used. It is expected behavior that the device could enter multi-monitor mode only on maximizing the session window in the device screen but not the external display. The behavior may be changed in the next release.

For the issue showing white content for multi-monitor mode. I think it is the not enough VRAM issue for the VM. Setting more VRAM for the VM should help, or you could try to reduce the client resolution, by setting "1/2 scaling" in the Horizon client settings -> display -> resolution.