When setting/editing the remote name, if you use certain invalid characters, the remote entry disappears without error/warning.

I had a remote repo entry called 'dotfiles/master' but that didn't specify which remote server it was using. I edited the remote name to be "GH:dotfiles" and SourceTree accepted it. Upon clicking the OK button, the remote entry disappeared.

SourceTree needs to warn or convert invalid characters to valid chars. It already does this if you use a space in the remote name, which is converted to an underscore.