Show, mandate field not working in Web desk

I have a window calc to show and mandate a field if the Category is set to 'Other' on an incident:

import System
static def GetAttributeValue(Incident):    HideOtherCat = true    MandateOtherCat = false    if Incident.Category != null and Incident.Category.Title == 'Other':        HideOtherCat = false        MandateOtherCat = true    return String.Format(":SetHidden(_OtherCategory,{0});:SetMandatory(_OtherCategory,{1});",HideOtherCat,MandateOtherCat)

At present, when I launch a new incident, the field is hidden and not mandatory.  When I select the Category as Other, nothing changes until after I save the incident, at which point the field shows and is mandatory.  This, essentially, allows techs to enter a category as other regardless of what the category actually is, and makes it the next tech's job to fill in the details of Other; the intent is to require our techs to enter details when they select Other, which should get them to just select the correct category the first time.

Am I missing something to make it work? Or is it working as expected?

I tried setting the calculation type to both Window Calculation and Before Save.