Are you selecting “Custom” when the bookmark pop-up appears? If not, I think that would be expected behavior.
I actually just can’t change that at all, until the field is filled out.
The only two things from experimenting with it just now that seemed really unexpected to me was that the date/time selector instantly got changed by the relative time picker, even when I cancelled my changes. (It doesn’t visually change until I cancel and reopen, but it does occur) Furthermore, if I manually blanked out the relative field before canceling, then it caused the picker to change to the current time.