Although this happened a few days ago, I thought I'd share perhaps to prevent anyone else from going through the same thing.
I spent over an hour double checking my code and having the program report the dimensions of the monitor and the window created. Previously it would auto-detect and fit within this area, and also center the window on the screen.
What was happening was everything the window created was too large. The content would go off-screen, the window wouldn't even center itself correctly anymore, and the reported window size was inconsistent with the actual dimensions when I took a screenshot and measured. It was like every value pertaining to x/y coordinates was multiplied.
Turns out that's exactly what the problem was. I had set up a new monitor to output 1080p and had unwittingly checked Windows "Make text and other items larger or smaller" and it turns out that applies some kind of transformation matrix over everything -- the Allegro program didn't even realize it, but some other programs seemed to adjust to handle it better.
If I had to guess most gamers wouldn't use that feature, but it was still disappointing to have Windows bully Allegro like that. Or maybe Allegro was just sick of it and not taking anymore crap from Windows