I dug a bit deeper and it appears that the IRS is aware that payroll vendors and software may have been rejecting Code P for civilians based on W-2 and W-3 instructions where it says the code is to be used only for armed forces personnel.  On that basis, the latest guidance is that Code P should not be used for civilian moves.  Qualified moving expenses described in Notice 2018-75 are still excludable from wages, FIT, FITW, FICA and FUTA but employers may choose to provide a separate statement to affected employees in lieu of using Code P.

https://www.irs.gov/newsroom/frequently-asked-questions-for-moving-expenses

Given this guidance, if Intuit will take some time to fix the problem (which they should do), I may consider ignoring the input for Code P and treat it as if it's not there (at least until Intuit gets this resolved).