Symptom
Trying to clear a brand field back to empty: 400.
"브랜드명은 제출 후 변경할 수 없습니다"("Brand name cannot be changed after submission.")
Empty → value works. Value → empty doesn't.
★ Put in the wrong value and it's permanent. Don't write on the assumption you can undo.
Side note: where I was wrong
I recorded "an empty brand makes approval stall" and then retracted it. Empty passes. The real cause of the stall was the previous entry's chained calls. Correlation read as causation.