This happens because GIB thinks you can still have 5S [and/or 5H!) for your 3N bid and simulates accordingly, instead of understanding that 3N limits you to 3S. This is a common GIB problem that has been reported many times and hopefully will be fixed sometime soon.
i am surprised u didn't open this hand....... I don't think this is robot reward.... (because there is no hand records for robot reward) If you first pass, GIB will assume 1s= 4s instead of 4th suit forcing.. (he assumes u can have 12 HCP au maximum) I met this problem in robot reward, then i changed my style.
You could have 0 spades for the 1S bid- just solid diamonds and some values in hearts. They should rewrite the 3NT definition as final contract with no definition of hand- if partner was interested in what GIB held he should have bid differently.