This was mentioned in T177485 because the problem started at the same time, but it is apparently a different issue, as that bug has been fixed but this one still persists.

Steps to reproduce:

1. Go to any item and click "Add statement" (or "Add value" on an existing statement)
2. Enter a property and a value (doesn't matter what datavalue type it is)
3. Press Tab to get to the "Add qualifier" link
4. Press Enter

As you will notice if you try this, this will add the fields for adding a qualifier as expected, BUT it will simultaneously save the statement.

This did not happen until the software update ~2 weeks ago.