Possibly relevant: suppose you are using single sign on through your LMS. Then the first time a student accesses WeBWorK through the LMS, if they do not already have a WeBWorK account, one will be created for them. And when an account is created automatically this way, all visible (meaning, set to "visible") problem sets (open or otherwise) will be assigned to that student.
WeBWorK Main Forum
Assign homework sets automatically
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