Re: Investigate the Strange Chalk Markings disappears from war table.

Apart from being in almost the same position, this actually appears to be by design, as it happens when you progress "Choose Successor in Lydes: Part 1". I went back to my save before that point (only an hour of gameplaty for me) and completed it before moving this Lydes mission set on.

It may also be a bug, but it is a repeatable issue.