I guess this is not technically a bug, so I categorized this as feedback.
The thing is, if the sidebar has tag #foo selected, I click on a note and remove #foo from it by editing its content, the note just disappears before my eyes and I am left with a blank page and a red caret. That is very disconcerting, where did the note go? Why? My unit of work is the note.
I believe it would be less surprising if you stay in the note. If UI consistency requires the selection in the sidebar to be updated, so be it. For example, if #foo was the only tag in the note, the sidebar focuses “Untagged”. If there were more, pick first one maybe, and make the sidebar show that one.