Snap to grid in PRV timeruler doesn't work at times

2017.07 and many earlier versions.
In the PRV, snap to grid doesn't work in the time ruler. I am not clear on the exact causes of this, it seems to occur most times immediately after opening a project, and sometimes randomly at other times.

This problem manifests itself as an inability to set the now marker at a grid point, aka clicking in the time ruler behaves as if snap is off, even when it is on.
 
Toggling the main snap settings in the control bar do not fix this. 
 
Opening and then closing again the redundant snap tool* on the far right of the PRV suddenly makes the time ruler respect snap, and all is good again. This is a sure fire fix every time.
 
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* why is this other snap tool here at all?