Please let me know why ?

MCB rated for 240/415VAC, 6Amperes trips on 5 Amps DC current at 24VDC voltage level

Ambient operating temperature: 32 ° C
MCB Designed for ambient temperature of 55° C


Why MCB should trip:confused: ?

Very possible that a loose connection, while the current is low, still, a loose connection will create a high resistance that can create heat at the connection, If this connection is at or near the breaker, the heat created will be conducted through the wire or components of the breaker and migrate into the breaker and activate the thermal component in the breaker,causing a trip.

The breaker must be rated to handle DC, also.

Let me add that you also did not say if the breaker was new, or how old the breaker was. As breakers age, they trip more easily and at lower currents.