If the value assigned to FEEDBACK plus the internal delays of the feedback path exceed the PERIOD, the correct Tdcmino value is not reported. The DCM never deskews more that one clock cycle, and the Tdcmino value should never be larger than one clock cycle.
You can work around this by subtracting one PERIOD from the FEEDBACK constraint; negative values should work for the FEEDBACK (never tested).
PERIOD = 10 ns, dcm2pad = 2.0 ns, pad2dcm = 3 ns, FEEDBACK = 6 ns
This reports the incorrect Tdcmino value. To correct this, make FEEDBACK = -4 ns. You will receive the same overall result, but the Tdcmino value will now reflect hardware.