Well, I guess I'm just talking to myself. I must be the only one running into this right now.
I did manage to fix it, but it's a bit of an ugly way to do it. I've compared the code for different frameworks in platformio, and I haven't really figured out why it fails on STM32, but not the others I've tried. No, I haven't exhaustively tried them all.
In the .platforio/packages/framework-arduinoststm32-maple/STM32F1/cores/maple/main.cpp file it's really almost completely empty. But then, so is the one from MySensors, though it has one addition for serialEventRun. Interestingly, other frameworks through platformio have this line as well, just not this one.
Anyway, the MySensors one has that extra, so I commented out the maple one through platforio, and then it lets the compilation complete. I haven't gotten far enough to know for sure that it doesn't cause further problems down the line, but I hope this is progress, and I wanted to get it written down while it's still fresh in my mind.