Heater core done again

Had my heater core replaced along with the thermostat last year because it wasn't functioning properly. The heat from the vents was coming in as waves of cold and hot.

Fast forward to today, heater core is done again. Vents are blasting cold air and hot air in waves.

I bought a cabin filter and installed it shortly after I replaced the first one as I had assumed the lack of a filter was to blame for my faulty heater core.

What could cause a brand new heater core to go bad within a year??