Cards with a "no preset spending limit" feature typically will not report the revolving limit. This includes VISA Signature and Mastercard World/World Elite cards, and that covers most "rewards" cards because these generate higher merchant transaction fees to fund the rewards.

Has your Merrill+ account been upgraded from Platinum Plus to Signature? Even if your physical card still states "Platinum Plus" the account may have been upgraded, and the limit is probably no longer reported.

You may need to look at Amex-network cards issued by Bank of America, Barclays or Citi to find a card that will report a limit. These cards are not issued or administered by American Express, but the charges are processed through their network rather than through Mastercard or VISA.