If I submitted Form 720 Spain in 2017 reporting accounts abroad, but then in 2018 my authorisation over one of the accounts was revoked, should I declare this loss of authorisation in Form 720 Spain?

Yes. The Tax Agency requires that you report all authorisation revocations. You must report the date of the revocation, the existing balance on that date, identify the account and business or company name of the bank and its address.