By design, only FXA and FXC are available for use at 88.2kHz and 96kHz for the RM mixers in UC Surface. As expected, FXB and FXD are hidden at these higher sample rates to avoid confusion.
When switching from a higher sample rate (88.2kHz or 96kHz) back to a lower sample rate (44.1kHz or 48kHz), a known issue prevents FXB and FXD from reappearing right away as they should. In order for them to reappear, it is necessary to leave the Settings Menu and then return to it once more after the sample rate switch. This bug will be addressed in the next maintenance release.