This bug has been in VA for a very long time.. so I figure that I might as well finally mention in. :)
VA's handling of active and visible code windows isn't quite right - it seems to only apply highlighting to the currently active code window and not all code windows that are visible.
The easiest way to repo this is to just drag the horizontal split plane down - the bottom (existing) part will have correct highlighting and the top part will had no highlighting until you click in it to make it the new active code window.
The same issue will happen if you have two files open and visible via a vertical split, then close & reopen the solution. The non-active window will not have any highlighting until you click in it.
On a side note, could this be related to the (many) VA bugs I get? It's been suggested by others here that since I'm nearly the only one who uses split planes (and I'm *always* using them) and I'm also the only one who sees so many VA bugs, that it seems that it may be more than just a coincidence. Perhaps something to do with VA's tracking of what it is actively "seeing", and split planes mess this up?