The Surface Pro 8's Dynamic Refresh Rate feature is only available if you manually download and install the latest graphics driver from Intel. Microsoft will likely add support in a future Windows update, but there isn't a specific date yet.
Update 03/05/2022: Now that the Surface Laptop also supports Dynamic Refresh Rate, you can edit your art in 4K on this device, without having to download the driver from Intel.
Before you install the new Lumion app, make sure you clear your recent apps by going into Settings, then tap Apps & features, then tap Manage apps. Clear the old version of the Lumion app from this list to free up some space.
Next, log into your Surface Pro 8 using the Microsoft, not lumion.com, login page. It should bring up a prompt that says Lumion, and not another app, like Photos or Skype. It asks if you want to update the Lumion app, and then says Lumion will be uninstalled. Press the Applications tab and scroll down to Lumion. Press / Volume Down to select Lumion, and press Enter to install it. After the installation completes, back to driving mode. d2c66b5586