I have 11.6.3 PAS and wWe found that the memory distribution of the eight instance of _mproapsv was not equally distributed in the PAS server. I am attaching screenshot of the usage of _mproapsv under Load.
Your image didn't come across, but there is no "load balancing" among agents within a PAS instance, only across PAS instances.
The session manager is designed to do the fewest number of context switches, so it will use the first available session in the first available agent, even if there are "spares" available.
We have set this to 16.