Not sure if this was ever resolved or not. I'm having the same issue on a Galaxy Tab Active2 (SM-T397U, Android v9, One UI v1.1), and I tried the Safe Mode test. The issue does not appear to present itself in Safe Mode - if the wallpaper is already "shrunk" before restarting in Safe Mode it still appears that way, however if I set the wallpaper again to full size I can't get it to "shrink" with any of the normal actions. For me usually the issue happens with things like entering & exiting apps, entering & exiting Settings, or entering & exiting app folders - one of those actions will randomly trigger the wallpaper going from full screen to ~1/3 of the screen. Note that my tablet is always locked in landscape orientation, due to a HW cradle used with the device.