American Express released an update (pdf) whereby new cards earning Membership Rewards points will by default be placed in a separate Membership Rewards account. You can manually pool the points together in the AmEx app or on the website using the ‘Manage Connections’ option. It might take a few hours for that to show.
Hat tip to michikade
Update 1/16/26: To quickly find the pooling option, you can login directly to Membership Rewards, see image below (ht Parts_Unknown):
