This is probably a case of "some people never get the message," or maybe "IRS didn't make the message clear the first time."  The W-2 instructions for 2020 required this "clarification":

Clarification on code P reporting in Box 12. The only amounts to be reported in Box 12 under code P are moving expense reimbursements paid directly to a member of the U.S. Armed Forces who moves per a military order and incident to a permanent change of station, which are excludable under section 132(g) of the Internal Revenue Code. Employers should not report excludable allowances described under section 912 of the Internal Revenue Code or any other amounts under code P.