This appears to happen when students join your class through a registered SSO login, such as when using their existing Google or Facebook account. I don't understand why Canvas devs thought the numbers should be put into student name, when clearly they have access to the name. Clicking on the numbers pops up a sidebar that shows the name, but this doesn't appear anywhere while grading, which is a huge issue.
Students can update their profile as a workaround, but there should be a LMS wide update to look for this and fix it.
Please don't mark this response as the answer, because I consider this a problematic, unresolved issue.
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