I have had to do this several times since the merger, at both Marriott and SPG legacy properties (but worldwide, not just Maui). Legacy SPG properties that didn't previously show this charge do now. For most of them, I had stayed before and was quite confident it wouldn't be an issue (i.e. they weren't going to charge for 2 children under 12 staying in existing beds). However, it does cause some unneeded uncertainty when booking unfamiliar properties, especially internationally.

Is there a blanket Marriott policy? Is this just another IT failure? I have seen other suspicious glitches on Marriott reservations, where a booking for 2 adults /2 children became a booking for 4 adults sometime between the reservation and check-in date.