Hello!
I’ve just started experimenting with my first font and stumbled upon a small problem.
When exporting my font including the weights Light, Regular, Medium, Bold and Black, the Light weight’s height acts differently to the other weights. I’ve checked all the values and it doesn’t seem like anything is different between the weights.
It is possible that in your lightest master, you have a glyph that reaches far below your baseline? Try the Test > Report Highest and Lowest Glyphs script in a my script repository. It can help find the culprit.