Are you using the textarea from CoreComponents? The docs call out that normalize_value does something around this. I’m not actually sure why this is necessary (of course not saying that it isn’t I just don’t know) and is something I’ve never encountered before. Probably a cross-platform thing? Would love to know myself!
But in Ash the action/changeset has no bearing, only the type. It is up to the type whether or not an empty value (like "") should be transformed into nil. This does mean that additional work may need to be done in the form/action if you want to both allow nil values but turn empty ("") submissions from a form into nil, but I think generally simplifies things.