Losing gradient filling format

- [x] Bug

I opened some sheets that have cells formated with gradient filling. Once I saved the workbook (even if I don't do anything else), the format is lost.

Reproduce the issue by opening and saving a workbook containing cells with gradient fills, then inspect the workbook-writing path for where that formatting is serialized. Done means the gradient fill remains intact after saving and reopening the workbook; the payload names no source files or tests, so locating the relevant code will require repository research.

Written by the indexing model from the issue text.