What does it mean when a system register like TTBR0_EL1 becomes unavailable?

In debug of our Linux boot, we often find that after we single step a simple instruction such as ADD x0, x0, #0x18, we can't view any memory and the IDE suddenly shows all of the TTBRxELn registers are unavailable. What exactly does it mean for these registers to become unavailable? Given that the MMU is active, we cannot view any memory, including the location of the ADD instruction up at 0xFFFFFFC0084411D0 (approximately). Any idea what could be going on that some, but not all, system registers go away like this?