For the first time, I've just tried enabling the option 'Always show panel selection dialog when opening connections'. The behaviour I saw was not what I expected – maybe there is room for improvement?
When attempting to open any connections, the only panel(s) presented in the dropdown were those that already existed with currently open connections. If no connections, and therefore no panels, were open already, then the dropdown list was empty/blank. Not even the pre-defined panel configured in each connection's config was available as a choice in the dropdown.
The panel dropdown would show the connection's own pre-defined Panel setting, as per what it has in it's Config panel, as the default selection. And then in addition to that, all of the other Panel names that mRemoteNG knows about, from all other connections (including 'General', whether or not any other connections even exist), plus any panels that are currently open that aren't defined in any saved connections.