ELCB or main incoming MCCB tripped? Different device tripping will signifies different issue.

For the ELCB, if tripped , it is associated with the earth fault . You will not be able to switch back the ELCB until the fault have been rectified. Possible fault may occured if the wires short circuited/ insulation damaged, water contact , equipment fault, melted cable due to overheating/ undersized cable , etc.

If MCCB tripped, it is associated with the over current / sudden surge of current . The lightning may be one of the cause or it can also signifies that your unit is drawing the power until there is no more buffer of safety, any small fluctuation (up surge) will cause the incoming current to exceed the MCCB rating & leads to tripping.

The new AC unit is fully inverter ? The AC is in operation during the lightning / trip event?

I have higher tendency to believe it is due to leakage / fault level from the AC unit if such is the case. You can totally isolate the power of the MCB to this unit in future when there is lightning, observe if the ELCB still trip. If the ELCB not tripping after the isolation of the power to this AC , you can pin point the issue to the AC unit. If it still trip, it is not the AC .

The reason I asked if the AC is inverter or not because for the inverter unit, there is frequency modulation to the compressor . Whenever there is the frequency modulation , the harmonic distortion will increases & may contribute to nuisance tripping.

You can safely eliminate this possibility. If there is fault, no way you can switch back the protection relay until the fault is rectified.

The more troublesome one will be the transient fault and this transient fault level got something to do with your AC unit.

It means, the trip is not due to the new AC unit, it is something else.

You many not be able to eliminate possibility of double extension until you try to isolate the case during the thunderstorm.

The last few days lightning during evening were bad.