As an aside we have backed up the database from this snap-installed server and restored it into the latest version installed manually, the problem has persisted, which i guess confirms this particular issue is in the database
I just had the same issue with the latest snap install. The message was shown to every user, when the URL changed the resources (pictures, video…) were not shown for anyone, and changing it from General would just revert it.
However, coming to the server and doing
sudo snap set rocketchat-server siteurl=<my url>
sudo service snap.rocketchat-server.rocketchat-server restart