Could you please describe the steps needed to repeat the problem on the screenshot in more detail?
When I open and add some text the fonts are fine.
Are you sure that the "Previously saved text" is not formatted as "Monospace"? Please try selecting it and pressing Ctrl+M several times.

I find the idea interesting, although a bit nonintuitive. I'm afraid people won't easily understand the logic behind this behavior (they may already forget that they've opened a note).
I have one more suggestion:
1) Always keep the focus on the last note on ESC
2) Use plain Backspace as a shortcut for "All"
What do you think?

When you click a tag on the sidebar, focus doesn't leave the notes list.

This would be useful if you've picked the tag from the sidebar, as opposed to entering it in the search box.
It also allows for unification: it can be made so that ESC will always reset the focus, while Shift+ESC would always try to keep the last focus.

How about introducing Shift+ESC to reset the tags filter while keeping the current focus?

I also agree that unification is needed.
I suggest that the position should be reset if the focus didn't leave the search box, and preserved otherwise. But I guess this won't fit the bill for liruicn.