Very simple: turn the CSS pipeline on. Breaks. Turn it off, works.
I went into the .zip install that’s been broken all morning and turned the CSS pipeline off in system.yaml. It now works.
The reason the .zip didn’t work and the github install did, is as stated, the .zip has the pipeline turned on by default while the github install does not. If I manually turn the css pipeline off in the .zip install, it works fine.