Isn't the first buffer always occupied by the validity data?
I'm not sure I understand this change

//===--------------------------------------------------------------------===//
ArrowArray *ArrowAppender::FinalizeChild(const LogicalType &type, unique_ptr<ArrowAppendData> append_data_p) {
	auto result = make_uniq<ArrowArray>();

	auto &append_data = *append_data_p;
    ...
	result->buffers[0] = append_data.validity.data();

All array types, with the exception of union types (more on these later), utilize a dedicated memory buffer, known as the validity (or “null”) bitmap, to encode the nullness or non-nullness of each value slot. The validity bitmap must be large enough to have at least 1 bit for each array slot.

https://arrow.apache.org/docs/format/Columnar.html#validity-bitmaps

Unlike other data types, unions do not have their own validity bitmap. Instead, the nullness of each slot is determined exclusively by the child arrays which are composed to create the union.

This tripped me up as well when I wrote the original implementation, but I guess most readers are pretty forgiving (except for the arrow-rs one)

Ah actually, yea this makes sense: https://arrow.apache.org/docs/format/Columnar.html#buffer-listing-for-each-layout