You have a PMOS input comparator. And a supply, VDD = 0.8V.
Now with input at 0.751V and 0.749V, your PMOS devices M1, M2, M7 and Mb are all dead. How do you expect your comparator to work with such inputs?
With the inputs at 1.2V and 0..3V, still one half your comparator is still dead. The |Vgs| is negative for the transistor with 1.2V input. The reason it could make the decision is that because it is now basically a simple Common source amplifier with the 0.3V input.
There is the Common Mode and Common Mode Range and the Input Differential Range. And I think you are having a confusion here.
Just because you can support a common mode of say 1V does not mean you can give inputs of 100V and -98V to a comparator with a supply of 2V.