Converging lines of southward longitude?

Long time reader here LOL, I got a question! I ask many people and get no answer.

How, on a flat earth, is it possible for lines of longitude to converge going southward?

When we measure the physical distance between lines of longitude, it matches the globe. They seem to converge going southward, south of the equator.

Are not lines of longitude straight and evenly spaced on a flat earth?

Since they are based off of the movement of solar noon along the earth, they have to be both straight and evenly spaced.

I don't see how it's possible to have straight lines converging at both ends.

All the flat earth experts I've asked just refuse to discuss it, maybe somebody in the community can explain it to me! Thanks!