Yep thats why I write I don’t know what siye of a cable you use ;)
but since I expected the cable to be very short (step down converter directly in front of the pi and the cable is probably open to all sides
VDE rules say you should use at least 0.5mm² up to 2.5A 2m
and at least 0.1mm² up to 1A 2m
So you should use 0.5mm² to be on the safe side.
0.25mm² should probably work aswell when you do not have long cables and do not stress the pi fully out to 2A ;)
But yes @MadScientist is right you should definetly not get below this
and keep in mind those values are for copper cables. If you use aluminium cables you obviously need thicker cables