Don't know what else to add, pretty annoying straightforward issue, create a curved line, duplicate it with control J, expand stroke, boolean, no matter which operator, does not work on it. Duplicating the expanded stroke and use Add on them does not work either...
Workaround: After spending too much time on it, if someone has the same issue, delete a few nodes and try it again.
Desktop 2021.08.11 - 17.34.57.01.mp4
charaxcter.afdesign