Gotcha, thanks Tippo.
Both last night and this morning it was colder than usual (-20*C ish) and the alt wasn't charging when I first started the car. I gave it a couple minutes to warm up but charging only came back after shutting the car off and then starting it again. It worked immediately after restarting the car. That makes me wonder if it's the alt causing problems or one of the other wires to it? Maybe this is a bad assumption though. Every start afterwards, when the car is at least a little warm still, it's totally fine.
It's hard to find details on how the regulators interact with the 3 wires plugged into it but from what I can tell the L wire is just for the warning light? I have a voltage gauge too so I know it's not just falsely triggering the light. The S wire appears to be feedback from the battery? And the IG wire looks to be power from the ignition switched common feed used to generate the initial magnetic field?
Since plugging and unplugging stuff seemed to fix the fuel pump power issue I'm tempted to just unplug everything in this circuit a couple times and see what happens. I hate intermittent problems to begin with, but I really don't like having to diagnose something that's only a problem well below freezing lol.