Hi.
I have the same issue with the single sim version of this product.
As i see this must be an LCD driver issue, and as i know there will be no solution for us

because this product is outdated

from all point of view. No more Win10 support (Phone issue fixes, driver fixes etc.) because the last supported device is the Lumia 640, so the solution for this is = Take the Phone into your left hand and throw away and never ever buy another one, because if they selling you a ****ty product, they fixing it but if it reach the 2 year oldness then yo must throw it away.
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I use the last official released Win10 14393.1358 but the issue is still there. Before the Bootloader loads the Driver there is no issue (Microsoft logo and the first Blue Window), i have made a video from this, but i know, nobody will work on this issue on the manufacturer side. These devices will run out of warranty and support time period or the are out now, so... i only hope, but as i see the future steps of the manufacturer, they don't care about those peoples, who has buy one product from them. Then get the money ant that is the only thin what matters to him.