Manjaro as distribution does not change user settings - that is understood as files residing in the users $HOME
I am guessing - without resent knowledge of Gnome - even though - it is possible the update of Gnome triggers a rebuild of the system wide dconf registry and that update then reflects into the user dconf.
That would be Gnome standard behavior and would be the same no matter the distribution.
That is a normal feeling - and if you are uncomfortable the rolling release - you are free to do what ever you want.
You are the system - updates are not forced - they are voluntary.
You can implement your own mirror of Manjaro packages and thus keep your system at a certain level - possibly forever - if that is what you want.