Hello forum.  I saw a similar thread but it was for a Roku Express different to my case.

Roku-3 480 mirror connection with Win 10 fails

When I go to Connect app in Win 10 it shows "Computer name" is ready for you to connect wirelessly.

Laptop has two graphics cards.  One for hdmi connections (NVIDIA not Miracast) and another for the internal display (Intel UHD Graphics 620 Miracast compatible).
Laptop has Intel Dual Band Wireless AC-8265 Miracast as Source.

Roku-3 480 device is detected successfully.  Connected on the same network.  

Roku is connected wired to router and laptop is connected wirelessly to same router.

ROKU  is mirror enabled, accept always, access permission default.  It is setup to update automatically so it should have a recent firmware version.

It connects for a couple of seconds then fails.  Display flickers when trying to connect.

Yep... Roku mirroring has been like that for many years.  If you want to mirror get a Chromecast.  You can thread after thread of broken mirroring.

On my ultra 4660x2 screen mirror worked fine till about beginning or mid-summer. Never after that. Different Windows PCs, Android devices do not do mirroring anymore. The Roku flashes the mirroring screen and then go back to the list. Tried different things and even Wifi connections but same issue.