The default (--vgdb=yes) valgrind gdbserver support is fairly lightweight, but might cause some registers and flags values to not always be up to date due to the optimisations done by the Valgrind core.
gdb is unable to show some parameters or variables on a breakpoint or when single stepping the program running under valgrind.
Workaround (if any):
Use valgrind --vgdb=full.
Better gdb debugging experience, but (much) slower program run under valgrind.
See also the upstream manual as quoted in comment #2 with a somewhat subtler workaround for this particular case.