I have exactly the same, really annoying problem with all my Win 10 VM's (with installed VMWare tools, in my case 10.305) after upgrading to Workstation 15.
I did install VMware Tools 10.3.2 before when I still had Fusion 8.5.10 and Workstation 12.5.9 but I can't say with certainty that the flickering didn't occur as it could be that I just didn't notice it. | vmx| I125: DnDCP: set guest controllers to version 4 | vmx| I125: DnDCP: dndGuestVersion from vmdb failed, setting to 4 | vcpu-1| I125: GuestRpc: Channel 1, guest application toolbox-dnd. | vcpu-1| W115: GuestRpc: application toolbox-dnd, changing channel 65535 -> 1 | vmx| I125: Guest: toolbox-dnd: Version: 10. | vmx| I130: PolicyVMX_IsDragDropAllowed: Drag/drop policy is allowed.
| vmx| I130: ToolsGetCopyPaste: Copy/paste allowed: 0. | vmx| I130: PolicyVMX_IsCopyPasteAllowed: Copy/paste policy is allowed.
I turned on debug and the flicker time roughly coincides with certain VMware log entries in relation to VMware Tools. I also noticed that the the flicker happens just before the VMware Tools icon appear on the notification area.
This led me to think it might not be related to the VMware SVGA 3D graphics driver during power-up of the VM. I then tried just a sign-out and sign-in again and the flicker still occured. I changed the hardware compatibility on the Fusion Windows 10 VM back to 12 and switched back to OpenGL and the flickering still occured. It also seems to happen only on the Windows 10 VM (not on a macOS VM and Linux VM). Considering that it the flicker also happens on Fusion, it led me to think it isn't the host graphics hardware or host graphics framework (Fusion 11 uses Metal by default while Workstation on Windows uses DX11 and the graphics hardware I have are very different Intel Pro 5200 on the MacBook Pro while Nvidia on Windows host).