Can you please check if this issue is specific only with the PP_MIMO register or with the Write16 () function itself? Can you please try this function on any other register and see if you are able to write?
The problem was neither with the P-Port or with the Write16() function. The main application SOC was big-endian and my commands to the ARM core were interpreted as little-endian. Once I changed the P-Port to operate on big endian, the problem fixed.