When switching into focus mode to edit the template part, it's missing after going back to the template using the "Back" button.

Step-by-step reproduction instructions

1. Go to Appearance > Editor
2. Open templates details dropdown in the header.
3. Click on "More Options" for the area and "Edit {templatePartName}
4. Without doing anything, click the "Back" button.
5. See that now the template part you were editing is missing from the page.

Screenshot

I tested with TT1 Blocks and TT2 themes.