I know and confirm the existence of this bug since Aerofly 2022 but I never reported it because I forgot. Good of you to remember. In fact, the LNAV and even the VNAV of the Boeing 787 turn off by themselves some time after takeoff and while the plane is climbing to cruising altitude. This problem has occurred to me several times. If you are able to reproduce this bug to show the developers I suggest you record a video as it is better for them to understand the problem. When you record the video reproducing this failure, make a simple edit to the video circling the LNAV and VNAV of the Boeing 787 in red at the exact moment when these functions turn off by themselves.
I don't know but I tried all the departures from Paris Today and didn't find a single Error. VNAV and LNAV don't turn Off for me. I think You're making a mistake somehow. It can't be that it works for me and not for You. Unfortunately, the Screenshot of You is not accurate and blurry. My Speed is 240 IAS (Autopilot ON, Autotrust ON) and only increases in the second 90°+ Turn. A Screenshot of Your exact departure procedure would help too. It's very peculiar though.
Maybe reinstalling the Simulator would help too?🤔🖖