To clarify, did you open cards and cancel rather than paying the first renewal fee? Are you regularly using the American Express cards that you already hold?
Consider the possibility that American Express is also looking at accounts you hold with other issuers. Are they seeing many accounts with no activity?
Do you have frequent paid Marriott stays?
I have always paid at least one annual renewal fee (when applicable) for AMEX credit cards. Other than the Blue Plus card, I regularly use my other AMEX cards.
My Blue Plus card is rarely used, and has had $0 balance for nearly 6 months. I could cancel that if it would help. I offered that to the customer service representative, but there weren’t any assurances that was the root cause. Elsewhere, I have a couple of Mastercards with Citi (Premier and Double Cash) that haven’t been used in a while. Those could be closed too if that would help. Maybe 3 cc total with not much recent activity…I have had 2 Marriott stays in the past 7 months, December and February. Before taking action to close or cancel a card, I was hoping to get a clearer reason for SUB denial.