Posted: Wed Apr 21, 2021 8:50pm
Your bank is completely wrong. Green residencia cards remain valid by order of Spanish government.
This lifted directly from Ministerio del Interior (translated)
"The current registration certificates and the cards of family members of EU citizens - these during their validity period - available to British citizens and their relatives residing in Spain are valid documents to prove legal residence in Spain prior to 1 January 2021, and benefit from the provisions of the Withdrawal Agreement.
From July 6, 2020, the beneficiary cards of the Withdrawal Agreement have been issued, the obtaining of which is not mandatory but it is recommended because it is a uniform EU model."
Here´s a direct link to this page, in case you'd like to print the Spanish version and present to your bank
http://www.interior.gob.es/web/servicios-al-ciudadano/brexit#Residencia