A 'standby' period can be used to notify you of a changed roster (in which case your standby ceases and any required 'rest' begins) or to give you a new duty without any 'intervening rest', which is what happened to you. In your case you were called out to operate 1 sector (MAD-BCN). On arrival at BCN your duty and FDP ended. You then had the required rest and commenced a new duty/FDP ('re-rostered' because that new duty was not on your ORIGINAL roster). What was your ORIGINAL duty for day 2?
I cannot see any problem with what happened, to be honest, unless your employment contract has some clauses which prevent it.