positioning of subscript or superscript minus

Created by: orangeplanet

A subscript or superscript minus sometimes (but not always) wanders too far from the target. It seems to especially happen when the neighboring superscript/subscript slot is occupied at the same time. Adding curly braces to whatever is on text line seems to negate the effect, but I'm not sure why.

See https://jsfiddle.net/s830g49p/