Preview doesn't respect the "records being browsed / current record only" selection. If you want to see just the current record, you can:
1) Find that record only, then preview.
2) Open a second window showing just that record and preview that (if you want to avoid losing your found set).
That will actually fit in quite nicely. I am going to put a button on the main asset layout to print the detail page. That script would have to select only that one record only anyway..
Preview is always of the found set because it doesn't include the Print setup parameters. It doesn't know you want to Print only the current record.