A little more info.
I dug out another PS1 of mine and it has the exact same issue, so I can rule out the console.
I also recorded my OSSC’s display in slow-motion with my phone when I power on the PS1, and it flashes random resolutions on the screen so fast. So fast, I had the use slo-mo in order to actually capture more than one of them. The resolutions it displays can be pretty random. Here’s one example I recorded: IMG_0259.TRIM.mp4