After using the default "ping-type" monitor for years, I switched to a custom StoreFront monitor a while back thinking it would be even better - and one day the monitor broke StoreFront traffic. I went back to the default monitor and it's been working great since. I've only ever used Net Profiles to steer traffic out of a different IP. I don't now if they could be in effect here, though. You might try setting up a quick NetScaler Gateway + StoreFront test environment and see where the traffic comes from, then compare the test and production configs.

I tired a net profile and it wouldn't change the monitor off the SNIP when using the buit in storefront monitor.