Haaaaa, I don't believe! Is this informed in some changelog? I didn't see. I think that this is bad, because is very difficult to maintain all MK in field updated. So, when some problem happens, very very problably mac-telnet will not work because probably MKs will have different ROS versions. Come ...

Same problem here. RB750 Gr3 with 6.45.3 (Stable) installed and bootloader upgraded too. After upgrade, I reseted RB750 to defaults with "No Default Configuration" and "Do Not Backup" option marked and after reset I've lost access by mac-telnet from a CCR-1009 with v.6.42.6 that ...

Same problem with mac-telnet login from RouterBoard here. Tested: RB2011 with v6.45.1 stable - login via Winbox mac address -> OK; RB2011 with v6.45.1 stable - login via another RB2011 v6.43.14 stable via mac-telnet - ERROR: "Login failed, incorrect username or password"; RB2011 with v6.45...