When you create a recurring task you set up a "template" what we call the "parent". This recurring task is visible in the Master List.
From this template tasks get created and scheduled into the correct days.
The tasks you see in your days and complete are not connected to each other. This means, changing one (adding a note or subtask to it), will not add the same note or subtask to any other recurring task.
How to add a permanent note to a recurring task
So if you want to add a note to a recurring task and have that note show up in all the instances of the recurring task you need to add it to the parent task (template.)
Either find the task in the Master List and click the pencil/edit icon to get to the recurring task dialog or use the recurring task list in the menu at the top right.
or by going to the ☰ → Recurring Items view:
In the dialog you can hover over the title at the top to reveal the note icon on the right. Clicking the note icon opens the note and you can edit it.
Don't forget to save the change!