Rendering freaks out when adding underscore to text

### Reproduced
1. Create new note, Markdown is on by default.
2. Enter the following:

> `* ([Apple](https://books.apple.com)) _about how it was to work at Apple during the original development of Safari and later the iPhone, the internal social and technical processes`

3. Add an underscore `_` to the end. And observe rendering freak out.

### Observed

| Step 2 | Step 3
|--|--
|  | 

Then, switch to another note and switch back. The renderer is now normal and with italic text. Then, remove the underscore. Observe that the renderer corrupts in this direction as well:

| Step 2 (alt) | Step 3 (alt)
|--|--
|  | 

* MacBook Pro (Retina)
* macOS 10.15.6
* Simplenote 2.2

Reproduce the Markdown rendering issue in a new note on Simplenote for macOS using the supplied text, first adding and then removing the trailing underscore. Start by locating the macOS note-rendering entry point, then verify that editing no longer corrupts the display and that switching notes is not required to restore normal rendering.

Written by the indexing model from the issue text.