It doesn’t look like changes to city borders in OSM are making their way into the outline on city map view in CS. If a city annexes a new area or extends current border, CS does seem to pick up the new nodes in these new areas; they just look like they are outside city limits because purple border line doesn’t include them. Perhaps similar/related to past reports of streets/nodes being outside city limits? 
Example: Durham, North Carolina (Citystrides | OSM)
In OSM, there are two areas where border was changed awhile back (OSM changeset) and they display in OSM, but not CS border.
For a closer look, new extension moved that long, straight-ish border to the right to include more of Olive Branch Road and added little rectangle area below it.
In CS, the new nodes did come for the new area; they just look outside the border. Example: Farm Pond Trail
Durham has had two updates (Feb 10th & March 13th) since the new update query work was completed in Jan. So I don’t think it’s a ‘need to wait for next update run’ thing.