mfrac linethickness causes render issues

Created by: andreeapescar

When linethickness is set, the fractions don't render properly, the line height is wrong ( <math><mrow><mo>-</mo><mfrac linethickness="1"><mn>1</mn><mn>3</mn></mfrac></mrow></math>).

I tried to render in multiple ways:

And this how it renders:

The ones that have linethickness="1" set don't render properly. (I observed that em is used to render that line.)

Is there any fix for this? Thank you!