In Zotero Standalone (latest version, running on Windows 7):

0. Make sure author field is set to display as two fields (the default)
1. Add new item or edit existing item
2. In an empty author field, click on "(last)" or "(first)" to edit a single field
3. Type the first characters of a last name already in the database (e.g., Ein)
4. In the dropdown, the full name appears (e.g. Einstein, Albert)
5. Click on the name you want (or navigate with arrow down and hit Enter)

Problem: Now "Einstein, Albert" is inserted into the single field you're editing (i.e. "last" in this example). When you hit Tab to go to the next field, or Shift+Enter to add another author, or hit Enter or click elsewhere to stop editing, it'll be saved this way.

Expectation: whatever full name you select from the dropdown at this stage would be divided across the two fields as expected. Yet somehow the field should stay open for editing so that it remains easy to use Tab to go to a next field or Shift+Enter to add another field.

I seem to recall that this wasn't always a problem.

Bonus bug: If you set the author field to single field, the dropdown won't show up even if you type in a name that you know is in your database, whether you start with first or last part of the name.