General Description: The RISING and FALLING constraint syntax only looks at the rising or falling edge flip flops in that time group. The time group may have other synchronous elements like RAMs or Latches, but the RISING or FALLING time group should only include the Flip Flops from that time group. The Problem is that the timing group contains the rising or falling edge flip flops, along with other synchronous elements (Rams, Latches) will be placed within the time group.
The Development System Reference Guide documentation, Chapter 4 states that the timing group(RISING or FALLING) must be a group that includes only flip-flops.