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Just FYI I found a solution to this:
Change your avatar in the lobby instead of doing 'chat self' and the avatar will stick to your account across devices
Just tagging on to this to say I've also had this problem for quite some time. After a while, my "chat self selected" avatar changes back to the one i previously clicked on in the default list. If the avatar is selected from the default list, it remains - but if not, it doesn't.