If you are experiencing an issue with your AMEX account becoming unlinked after you have set it up, please check the MFA (multi-factor authentication) settings in your AMEX account. If you have the setting enabled to force a OTP (One time pin) on every login, the account will become unlinked after you log back in. You will need to disable the force OTP after every login in order for the account to stay successfully connected.

This is a known issue with Plaid and AMEX and they are working to expand the functionality.