Looking further into this now I have returned from France, please see my comments below.
Referring to the Complimentary Technical Information guide, full selectivity between the two MTZ1 breakers is not achievable, Trimble will not take into account the energy based selectivity, just time and current.
Full selectivity will be achieved with an MTZ2 2000A and NS1250 with the ii setting off. If the Ii setting Is set to 15xIn then you will achieve partial selectivity up to 30kA.
Do you know the full load current for the 1250A device? Full selectivity can be achieved by using an MTZ1 1600A and an NS1000L.
Please let me know if you would like to review the above maybe over teams, I am back in the UK so I will be easier to contact now.
Another solution would be to use ZSI (zone selective interlocking) between the 2 ACB's in question.
I have attached the information on ZSI for your reference. I have also sent a question across to our team in France regarding the selectivity between these two devices to confirm, I have modelled this on our own Electrical Calculation tool and I am seeing the same result as you, but as the guide states it cannot be achieved, I would prefer to check this.