I’ve seen a number of issues related to VNAV on our 787 fleet, including an uncommanded descent in the cruise and the amber lines through VNAV with an autopilot EICAS caution.
Follow-up suggested that it was caused by a software error on one of the GPMs, resolved with a reload.
I suspect that there are effectively two different autopilots on the 787. The “normal” one which is hosted on a virtual machine and handles all flying other than auto lands.
For autoland I believe the FCE computers take over with a much more robust and proven design (you can feel the clunk in the controls when LAND 3 or LAND 2 shows on the PFD).
This would explain some of the erratic behaviour of the autopilot in climb / cruise / descent phases.
I would be interested to learn how it actually works if any Boeing engineers read this forum.