The switch's mac address shown in the Mist Portal does not match the local Bridge ID displayed on the command "show spanning-tree bridge" output.
root@EX2300# run show spanning-tree bridge
STP bridge parameters
Routing instance name : GLOBAL
Context ID : 0
Enabled protocol : RSTP
Root ID : 32768.54:4b:8c:16:79:10
Hello time : 2 seconds
Maximum age : 20 seconds
Forward delay : 15 seconds
Message age : 0
Number of topology changes : 0
Local parameters
Bridge ID : 32768.54:4b:8c:16:79:10
Extended system ID : 0
root@EX2300# run show chassis mac-addresses
FPC 0
Base address 54:4b:8c:16:78:cc
Count 75
root@EX2300# run show interfaces irb extensive | match hardware
Current address: 54:4b:8c:16:78:cc, Hardware address: 54:4b:8c:16:78:cc
{master:0}[edit]
root@EX2300># run show interfaces extensive | match 54:4b:8c:16:79:10
<no output>
root@EX2300> show ethernet-switching table | match 54:4b:8c:16:79:10
This is the expected behavior. The Junos OS will grab the same chassis mac-address and change it’s last octet or its last two octets and then assign it to the local bridge ID. This newly generated mac-address does not belong to any of the interfaces living in the switch, it’s simply used for STP.