Look, in your first circuit you are tying the gate of M18 to VDD.
In principle, this is not the correct way of biasing because you want M18 to behave as a current source and create a virtual ground for balancing the current at each branch.
Basically, what you are doing is dropping the drain terminal of M18 to ground, so there is no Vds, then it operates in the linear region and not in saturation and the rest of your circuit is not operating as a consequence of this.
What you can do is to put an ideal current source, set the proper value (you need hand calculations for that) and mirror this current to M18.
This is the correct way of biasing since you ensure that your differential pair is drawing the current set by your current source and so you don't have floating nodes.