Posting here for a solution to a similar but not exactly the same issue. My issue was that when trying to add a point in an automation lane with write off, it would not add the point and the track would flash red. I could drag the mouse and it would behave like it was still in write mode. This affected every automation lane in the project, not just the one that was causing the issue. The bug was specific to a single project, but any project I opened that had the problematic track in (previous recent versions) would still have this issue present.
Anyway, disable track/enable didnt work. Restarting project/cubase/mac didnt work.
What worked for me was disabling the track where I first saw the problem, then importantly, adding a point to another automation lane on another track. Then I re-enabled the problematic track and I can add points again, bug gone.