The attribute CLKFX_MD_MAX allows the user to specify the maximum ratio of M & D values which can be dynamically programmed so that timing analysis may apply this to the PERIOD constraint's Clock Uncertainty value in the timing report. In 11.2, this attribute is not being used and the specified M & D values are always applied.
When is this going to be fixed?
This is a known issue. This is fixed in 11.4.