The customer is using Apstra controller to manage their fabric, 

During the upgrade procedure, the customer removes all existing configurations and loads very minimum configuration (pristine configuration)
they report that after each upgrade the interfaces snmp indexes are changed and they are not preserved 

This is expected behavior if the customer is removing all the interfaces related configuration , reboot and then push the configuration back 

If the interfaces are removed from the kernel and configuration ( which happens here after pushing the pristine configuration ) the snmp indices are released. 
Due to this, there is no guarantee of assigning the same SNMP index to the interface if there is a reboot.

There are reserved indices in the range of 0 to 500 which are static and always written to the index file, those remain the same always across reboot
The unreserved indices (beyond 500)considered dynamic indices have no guarantee of being assigned to the IFL/IFD as always over reboot.

In this scenario, loading pristine configuration removes all the logical interfaces and also some of the IFDs as well (like ge-0/0*). 

Due to this the indices are released and not written to the index file. Following that there is a config load and reboot which causes the reassignment of unreserved indices from the free list. 
This might assign new indices to the interfaces as it is not aware of the old indices before reboot.