Just a clarification first, in a notebook focus of the _tab_ is different to focus of the _contents_, and is different to the _current_ tab which is the one displayed, whichever has focus (tab or contents). IIUC not all themes distinguish focussed tabs from current tabs so the difference might not be clear.
Please be sure which you mean, in this case I will presume you do mean the symbols tab has focus, not the contents, and you actually want the contents of the editor to have focus, not the tab.
The results as described appear to be deliberate, the focus only shifts to the newly opened editor widget if the widget which has focus or would have focus if the window was focussed, is either of an editor widget or the symbols tab contents.
This makes sense since the open request is asynchronous, it can happen at any time, its not within Geany. In your case it comes at a time when you are not interacting with Geany, but it might come from a background process while a user is using Geany, and there could be a dialog open in Geany or some other partly completed interaction.
So to avoid causing issues by interrupting something (interrupting is rude and Geany is totally polite ;-) only if the focus is on or would be on a widget that will be changed by the open is the focus shifted to the newly opened editor.