mstatus.FS cannot be set to (0)'b01 or (2)'b10

HiFive Unleashed
1

Hi
It appears that the mstatus.FS field will not accept the values 1 or 2, the only values that appear are 0 or 3

whenever writing FS=1 or FS=2 the value read back is always 3

Is this correct, and is this documented ?

Thx
Lee