Grid exports columns as 0 width unless Grid column widths are specified in px. This is a terrible experience for users, and specifying grid width in px is terrible when you have users that can be using phones or 4k screens. Managing Grid for different screen sizes is already hard enough!
Solution 1: (Good)
The grid should export a default (64px) width for columns that aren't specified in px. Inexperienced Excel users are lost when columns are collapsed, but can easily resize columns as needed when each one is visible!
Solution 2: (Better)
Calculate a width - like [(rem value)*10]px. Even if it isn't perfect it will go a long way to creating something visually pleasing on initial open in Excel.