When analyzing the clock skew, the PRIORITY keyword is ignored in the clock skew calculations. Timing Analyzer is selecting the incorrect common node for clock skew calculations.
When is this going to be fixed?
This is only an issue for designs where the priority of the clocks is specified. The nearest common driver skew calculation always finds the common driver nearest the two loads for a particular skew calculation, but does not take into account which clocks might have precedence with the PRIORITY constraint for muxed clocks.
This is a known issue. This issue is scheduled to be fixed in the next quarterly release of the ISE design tools.