Clock skew from using global and local routing

Assign the pin to a global in the assignment editor. That should force everything onto the global, which is what I assume you want. (Or assign Global = Off, if you don't want it). The one case I mentioned where I know of demotion involves gated clocks, so this may be the cause. (If you can get rid of the gated clocks and use clock enables or something, that would be great, but naturally understand it's not always possible).