You should check what Tesla uses for battery module coolant distribution. It would probably be a good idea to use that as a guide, since you are using their modules.My Tesla modules have 5/16" coolant outlets.
Automotive grade coolant pumps typically have 3/4" inlets/outlets.
Is it ok to hook 5/16" line up to a 3/4" pump?
Seems to me you might have fluid starvation from the smaller feed line.