I expect that part isn't working, and isn't easily fixable... This is a tablet and some tablets commonly have a native screen orientation of portrait so that's probably what Plymouth is using.
When the gnome-shell login screen starts it detects the correct orientation and automatically rotates to landscape. So the issue sounds like Plymouth not supporting sensors/screen rotation. Not a very common issue...
I expect that part isn't working, and isn't easily fixable... This is a tablet and some tablets commonly have a native screen orientation of portrait so that's probably what Plymouth is using.
When the gnome-shell login screen starts it detects the correct orientation and automatically rotates to landscape. So the issue sounds like Plymouth not supporting sensors/screen rotation. Not a very common issue...