Greetings, I too have been frustrated by the loss of a single day. Here are some things I always check, most of which I know you know:
Resource time off during period in question?
Company holiday during period in question?
Task start and end time of task align with work time in calendar?
Your time zone same as time zone (schedule) of project?
In my experience, if my computer is set to PST, and the project schedule is set to EST, whether by intent or default, then if I enter a start time of 08:00 AM, that will start the task at 11:00 AM "project time," and the task finish will push a day.
When troubleshooting, enforcing the predecessor will sometimes force an error that reveals the issue.
~Jeff Jeff Rieth