This is Photoshop 22.1.0, on macOS Big Sur 11.0.1, on a MacBook Pro (15-inch, 2016) with Radeon Pro 460 4 GB / Intel HD Graphics 530 1536 MB.
If I resize a Photoshop window across multiple monitors, at a certain point most of the image disappears. Then if I keep making the window bigger, I see two copies of the right edge of the image, with a big gap in between.
Here's a screenshot showing what happens as I increase the size of the Photoshop window across two 2560x1600 monitors.
For what it's worth, the bug seems to depend on the size of the image area: if I expand panels to take up more horizontal space, I can make the window wider before the image cuts out. The maximum width before the image cuts out also seems to depend on the horizontal monitor resolution: if the monitors are rotated from landscape to portrait mode, where it's more common to size windows across multiple monitors, the image cuts out at a narrower width.
This seems to be pretty easy to reproduce. I can see this bug when using multiple external monitors, or with only one external monitor and the built-in display, and it can happen whether or not the external monitor is in high-DPI mode.
I don't know what version of Photoshop this started in happening in, as I just upgraded from an old non-cloud version of Creative Suite.
Disabling "Use Graphics Processor" in Photoshop > Preferences > Performance does work around this.