From your explanation, what you are trying to do is something like this
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The output common mode i a Vgs above ground. In this unity gain configuration the output common-mode voltage tries to be Vgs above ground because there is nothing that will pull the outputs to 0V. Because of the connection to the inputs, you will have Vgs across M1/M2 and the current source Iss. Which means the tail current source will most likely be in triode since it doesn't have sufficient headroom. So, the current in the diff pair reduces, maybe all the way to nothing. In this case the output common mode voltage will be at Vth of M7/M8. Question is, why do you think it is 0?