Two things needs checking here.

1. Is the battery being charged correctly from the alternator ?

2. Is there a residual drain on the battery when the car is left unattended.

Cannot understand why the dealer has changed two batteries without investigating the cause of the issue. Any self respecting mechanic or auto electrician should be able to do both the above. If you are not going to investigate the issues your self I would find another dealer or auto electrical expert to check out the above.

If you are going to investigate then to check if battery is charging correctly connect a voltmeter across the battery with engine running. The voltage should be around 13.6 to 14.2 V DC. Check AC volts, should be zero, if not rectifier is short circuit.

To check residual drain connect an ammeter between battery negative post and earth lead. You should only see a few milliamps for a clock or alarm etc

Is this the old shape or new model fiesta? This will be using a smart charge alternator, so testing the system is more involved , as charging voltages can go up to 15.8 volts .

Its the new shape Fiesta

My wife's Fiesta (also Titanium also 2017) has twice had a completely flat battery after being left for 2 weeks. On the first occasion there was a dashcam plugged in to accessory socket. On the 2nd occasion we ensured the camera was unplugged before the car was left. Two different dealers say battery is OK. Current drain measured by second dealer at 0.02A. Even if the battery was at 50% charge there is no way it would be totally flat (it would not even light up the mechanic's LCD meter) after 2 weeks with this current drain. Car works normally (including stop-start) if not left for long period and is not used for short journeys.

Second dealer suggested we bring the car in if we are to go away so that they can fully charge battery before car is left.

Clearly something is amiss but dealers are unable to find a fault.