On MX series router after upgrade to JunOS release 23.1R1 the MAC address assigned to physical interface can be different from MAC address assigned to this interface before upgrade

On MX-Series routers pool of MAC addresses is programmed into chassis midplane EEPROM. Exact pool size depends on product and can be checked from CLI:

user@device> show chassis mac-addresses 
MAC address information:
 Public base address    38:4f:49:8b:08:b8
 Public count           1984
 Private base address   38:4f:49:8b:10:78
 Private count          64

The software assigns MAC address to physical interfaces from public pool. After JunOS upgrade to release 23.1R1 and above the MAC address on physical interface could be different from MAC address on the same interface before upgrade.
For example on MPC2E NG HQoS card, if number of interfaces remains the same during boot, MAC addresses before and after upgrade change in the following way:

1. interfaces on PIC 0 will have the same MAC addresses as before upgrade
2. interfaces on PIC 1 will have MAC address less by 2 in last octet from what it was before.
3. interfaces on PIC 2 will have MAC address less by 4 in last octet from what it was before.
4. interfaces on PIC 3 will have MAC address less by 6 in last octet from what it was before.

On certain platforms, the assigned MAC addresses may also change if number of interfaces during boot is not the same as it was during the last boot.

This behavior is harmless. After upgrade remote part will be able to resolve IP address assigned to interface to new MAC address and communication should work properly. The only one situation when it causes problem is when remote side has static MAC to IP mapping. In this case incoming traffic will be dropped on MX router with a reason 'Input DA reject'. This condition can be checked from CLI as:

user@device> show interfaces <interface_name> extensive | match "Input DA rejects"
  Input DA rejects         XXX <=== non-zero and increasing if there is an incoming traffic from the peer with the static MAC to IP mapping.

If static MAC to IP mapping is configured on the peer device with please consider changing this configuration right after upgrade. Alternatively, a static MAC address can be configured on MX side:

set interfaces <interface_name> mac <mac_address>