2nd Destination "eaten"?

I'm having some trouble figuring out exactly how the destination filter works and how to keep it from "eating" the 2nd destination of the day.

I might use the 1st destination in the morning to go in my desired direction and that works OK, and I make sure to turn it off in less than an hour or after I accept a long trip in my desired direction.

I check back on the destination filter screen later in the day and sometimes I still have the 1 destination available, and sometimes it's already gone and there are none remaining.

Anyone know what's going on? Market is SoCal.