This is expected behavior as the MX8116n has a probing mechanism in place to detect the kind of optic or cable connected and then configure it.
The MX8116n FEM can operate at 25 GbE and 100 GbE. The 25 GbE solution can support both dual and quad port NICs, while the 100 GbE solution is dual port only.
The following sections describe the port mapping for each port mode of operation, showing an example of interface 1/1/1 and interface 1/1/31 on the Z9432F-ON to connect the MX8116n. The interfaces that are used are arbitrary for this example and are not a recommendation for connection order.