Solved as above in the question itself. Copying here to be able to mark a solved solution and flag the question as having a confirmed solution. Thank you.
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Auto rotation worked in live session but not after full installation [SOLVED]
I have first tested Ubuntu in live USB session for auto screen rotation and found it working. Both in landscape & portrait modes. But after full installation, it's not working. My machine is Lenovo Yoga 520 convertible. Can you please help?
It's all working after I switched to xorg from the gear icon on the login screen.
It wasn't working in Wayland, which was the default.
From what I read on the internet, it appears there's a bug in Wayland, which prevents auto rotation of the screen.My touchscreen tablet PC is a Panasonic FZ-G1 MkIII ToughPad.
I switched from Wayland to xorg at the login screen, via the cogwheel at the bottom right hand corner.