To put it simple: Even though Pacu visibly reach 100% reproduction, the number resets to 0% without an egg being laid and without reason, the NoPacuEgg.sav should show it well. The Pacu there is also be able to lay another egg before dying but it won't.
A bit more I observed and would like to note: First, I see only wild Pacu die without leaving eggs, then I get to a Pacu in time, feed&tame it yet even though it should have laid many eggs by now, it did never lay even one.
I'm not sure what is wrong and it might be no real bug but if the habitat is too small, and so would be pretty much all natural spawn points in return, then it is not shown (and should be changed anyway). Or if it is because it is Elderly... that's NG either (I know for sure that the value was @30% when just tamed).
0: Observe wild Pacu go extinct in the natural habitat 1: Quickly tame a wild Pacu by feeding it 2: Wait for the Reproduction value reaching 100% (<=the .sav is @99%) 3: Watch it reset to 0% with nothing more happening.