Typically this will be charged at the end of the first billing cycle. I do not have any direct experience with Corporate cards and cannot explain exactly what they are doing, but it is unrelated to the basic card fee.
Ok then, by what you are explaining here is what I'm understanding:
$52.50 credit - This is a prorated refund of my original $90 fee paid for my corporate account
$90 fee - This (I'm guessing) is essentially renewing my corporate card annual fee so that it lines up with the renewal time of the corporate platinum $450 fee. This kind of makes sense, though I don't see why It's necessary that it needs to line-up with the Platinum fee.
What I DON'T understand out of this is... WHY did they put the corporate card annual fee onto my Platinum card?! It's entirely probable that employers may pay for that fee, but not if it's on my personal card and not the one it was intended for....
$450 Platinum fee - This is expected to be tacked on at the end of my first statement.