@GrahamvdR this was a bug. What you’ve done is correct. Manager allows to use tax code on inventory write-offs becase in some circumstances, write-off does affect your tax liability (or credit). E.g. when the item is written-off for private use.
Tax Summary report wasn’t able to pick up these taxable inventory write-offs. The latest version (20.2.83) is fixing the issue.
Just to clarify, inventory write-off is not just for spoiled/damaged goods, free samples or internal consumption. It should be used for private use as well.