Math: Spacing in stacked fractions

Hi

Can anyone tell me why there is so much space between the fraction bar and the denominator?
I’ve set the spacing to 0% but that doesn’t change anything.
Any ideas how I can reduce the space?
Untitled 3.odt

Thanks!

Because you use a lowercase letter a, and the space includes the Cap height. Put uppercase A next to the small a to see the difference.