S button beside "placement" property of fingering resets the property
The "Set as style" button on String Number Placement rests to the default style instead. In Format > Style... > Text Styles > String Number there is no option to set the Placement.
The workaround is to select a String Number in the score, bring up its menu, Select > All Similar Elements, then set the Placement.
1) Select a note, open the fingering palette, double click circled 5; the string number is displayed above the staff by default.
2) In the Inspector, scroll down to Fingering, confirm style is String Number.
3) Change Placement setting to Below; the S button becomes active.
4) Click the S button, the Placement reverts to Above.
5) Confirm style was not set by adding string number to another note.