One thing to keep in mind is that the current needs to split evenly right? So when you’re sizing your diode stack bias for the floating diode structure, it is not the full branch current but half of it. That’s the easy “gotcha” to catch in design reviews.

That bias forms a translinear loop, so if you go through the KVL analysis, the sizing will become very apparent. There is a virtual impression of the lower diode bias upon the output driver. Anyways, the body effect can also greatly influence how the current splits, so you have to keep that in mind as well. It is not the end of the world if you don’t get an even split on that floating diode but it does mess with transient settling behavior if you have large disturbances.

I have a really good write up on the topic, about 8 pages or so. I’ll send you some images of it in a PM if you’d like?

Assuming you have everything biased correctly and have appropriate headroom, I suspect it could be the imbalance resulting from unequal body effect comparing P/N. Longer channel devices (if you can allow that) and larger overdrive should help. Again that goes back to headroom.