We do not have this level of specificity. If American Express' algorithm finds something it doesn't like about your qualifying spend it will withhold or retract the bonus. When you contact American Express the front line customer service agents will manually review your spending, and may offer an explanation, but it will be a guess. They do not actually know what triggered the algorithm. They can submit the claim for review by specialists who will make the binding decision, but cardholders do not communicate with those people.
One question: Is paying legitimate expenses (such as residential rent, tuition fees) etc. using Plastiq generally safe and not be misconstrued as MS?