Ooh, I found an autosave from before it started happening too.

Ah yes, that's probably part of the problem - the upper heat exchangers do indeed have no water (luckily I've not actually need the full output of this plant yet), so the turbines couldn't be running at full capacity (well, 91.6% capacity - 48 exchangers make less steam than 90 turbines can use ...

I rebuilt the entire block with bots (not from the spidertron), and now a different set of turbines is disconnected. It appears that non-functioning turbines can allow steam to pass through. The lower-centre section has a functioning turbine, then two non-functioning, another functioning, and then ...

What did you do? / What happened?

Earlier in the save, I built a turbine array as in the screenshot below. It worked fine for some time (I don't know when it stopped working, unfortunately, but I didn't notice it being a problem until just now because my Nauvis factory was quiet while I was off ...

Is it that the panels themselves don't respect the solar bonus/penalty or is it that the UI doesn't? It feels like on Vulcanus the bonus applies, though the armor UI still only says 30kW/panel.