Yes, this bug is known, and it slipped out since 2.4.0 was released in a hurry. I plan a minimal 2.4.1 release to fix this and a couple of other annoying bugs that didn't make it into 2.4.0. So sorry for the inconvenience. For now there is a workaround, but it is somewhat unsatisfying: use the checkboxes instead of the "Assign to All Current Users" button.
WeBWorK Main Forum
WW 2.4 "Assign to All Current Users" bug
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