If that’s the way the 787 is working for you, then you’re either lucky or the aircraft is bugged.
Modern Boeing LNAV is pretty consistent: to engage LNAV on or immediately after takeoff, the aircraft heading has to be within something like 20 degrees of the first waypoint on the route. Depending on the route you’re flying, that first waypoint may be a full 180 degrees away from your runway heading on takeoff. Typically this will show in the LEGS page as a vector. IRL, ATC will give you a heading to fly on takeoff once you’re wheels up and climbing, then vector you around or send you direct to a waypoint. Once you’re generally pointed at that first waypoint, either by self-vectoring or from ATC instructions, you can engage LNAV.
Anyway, FWIW, I made my first complete fligth in the 787 MAX in FS2024 last weekend. The plane has a TON of UI and usability bugs, and more than a couple systems-related issues. But LNAV and VNAV worked okay once I got the plane onto the planned route. (*)
(*) I did have an LNAV-related bug - ATC vectored me to a waypoint further down the SID than I had planed ahead of time. I tried to do what I can do in any correctly-working Boeing by doing a direct to that waypoint. The FMC in the MAX didn’t want to do that and failed to delete the earlier skipped waypoints. I finally just ignored ATC and flew the full pre-planned SID. But again, once I got onto that route, LNAV followed it fine.