I doubt that they would have changed this. A month or so back when Marriott just released their Premier Signature CC with its 15 nights added, caused a problem when it added more like 50 nights. A lot of people were upgraded to Platinum for a short while because of their rolling nights went above 75 (at least until Marriott fixed the problem. )

My understanding of this benefit is that they have specific times when they check this and I forget whether it is once a week, 2 weeks, or monthly that they check your balances. A search in this forum should find it. But if you recently made it past the 75 nights, it is possible their "check" time hasn't come around yet.

Your best bet is to call the Marriott Rewards support line. They know about this benefit and can even tell you how many nights within the next X days you will need to hit your next level. So they can tell you if you have enough to reach Platinum, or if their numbers and yours don't match yours.