In issue tracker import, updated_on for issues is required, although documentation says it is optional or nullable

On the confluence documentation at https://confluence.atlassian.com/pages/viewpage.action?pageId=330796872:

updated_on
The timestamp of the most recent change to the issue's state. This includes everything from assignee to status. A string in ISO 8601 datetime format (for example, "2013-04-21T03:12:04.660869+00:00"). This string can be null.

When I attempt to import an issues file with updated_on omitted, or with updated_on null:

Issue updated_on: Required datetime field not found: updated_on

I have several keys (I just noticed them in my E-mail) for this problem:
f40e5f7f-64e1-4fe8-b545-67d53a73e571
b654650c-e5b8-43fd-898d-74cba25af7e7
fecac5e9-438b-4277-9f7c-0bccf16e3d1e