Problem:

Keyboard layout reverting to US on (every) reboot

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Cause:

I found the same problem, but only on my wife's account. I'd set her keyboard layout to UK through Gnome preferences and every time the US layout would reappear and be selected frustratingly.\
It turned out that on the gdm login screen her choice of keyboard layout in the discreet option bar at the bottom of the screen was set to US. That was then overriding the Gnome layout preference.

Solution:

Selecting the appropriate keyboard layout on the next login solved the problem and GDM remembered the preference for next time.\
This isn't what the user would expect to happen. Is there a way to unify / synchronise the GDM and Gnome keyboard layout preferences ?

Reactivate Login Screen:

Go System → Administration → Login Screen and choose «Show the screen for choosing who will log in».