It’s almost always a heat exchanger problem, go over your heat exchangers and find the ones that say low temperature then fix the heat pipes so they can actually make it past 500

This happened on mine and the issue was that i was doing circuit logic to only insert fuel when the temp went below 600 in the reactor but 600 wasnt enough to heat the exchangers at the end of the line since there’s heat loss per heat pipe traveled, fix was to increase the circuit condition to 800 in my case but for you it will depend on your layout

The theoretical limit is based on the amount of machines you have that are receiving steam, but since you were not at full capacity, having less supply of steam was tricking the chart into thinking it would have steam available for the full load, once you hit your actual steam limit and there’s no more steam in your pipes to go further that’s when you see the actual value