I moved my hearth fire - and the old one remains. What's worse, the old one had been placed to trap my cabin, which I intended to give to a friendly newb. The newb is now trapped in the cabin. She can log off and go back to her own hearth fire - but I had left her some tools, which she cannot get to. (I'd advised her to put her own hearth where mine had been, leaving the stuff inside. She walked in with the material to build it, and found this surprise.
The game treats my new hearth as mine for purposes of logging in at my hearth fire; I checked.
Follow up - the old hearth fire appears to have become an unlit normal fire, which the new tenant has now lit. Hopefully she'll be free to take possession when it burns out. Maybe this is not a bug.