Custom expandIcon doesn't work in horizontal mode

[Nightliss]

Hey there, as in title for some reason expandIcon doesn't insert its contents to dom like on vertical mode. Is that intentional? I don't see anything in the docs which would suggest that behavior.

CodeSandbox link: https://codesandbox.io/s/broken-sky-4fewl?file=/src/index.js

I used customIcon example, please forgive me for no styling :)