The custom number formats are not parsing as relating to datetime values. Unfortunately, Excel treats datetimes as numbers unless the formatting is right and we use the same logic in openpyxl. I don't know what created the file but saving it Excel resolves the problem.
It looks like the uppercasing in the format is what's tripping the detection up. This is easy enough to fix but I wonder the formats should also be normalised to lower case as Excel seems to do.