STP Loop issue on long distance link

Question

I have long distance connectivity between west coast to east cost datacenter via our ISP network as you can see that in following diagram (BSN=Boston, PHX=Phoenix)

Now when ISP-A or ISP-B doing any maintenance during that time PHX-SW1 switch putting interface in LOOP* (ofc because BPDU is missing during ISP is down but curious why its thinking its loop and trying to protect link? is there anything i should be doing for long distance STP tuning?)

PHX-SW1-config:

interface Ethernet1/1
  switchport mode trunk
  switchport trunk allowed vlan 4

This is the condition when ISP-A is down

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Eth1/1           Root BKN*4         128.1    P2p *LOOP_Inc
Eth1/2           Root FWD 4         128.2    P2p 
Eth1/3           Altn BLK 4         128.3    P2p 
Eth1/4           Desg FWD 4         128.4    P2p 

PHX-SW1# 2021 Aug 31 21:02:10 PHX-SW1 %$ VDC-1 %$ %STP-2-LOOPGUARD_BLOCK: Loop guard blocking port Ethernet1/1 on VLAN0004.

when ISP-A is up it unblock interface and everything back to normal.

Update - 1

BSN-SW1 and BSN-SW2 is in vPC

BSN-SW1

BSN-SW1# show spanning-tree vlan 4

VLAN0004
  Spanning tree enabled protocol rstp
  Root ID    Priority    4100
             Address     0023.04ee.be01
             This bridge is the root
             Hello Time  2  sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    4100   (priority 4096 sys-id-ext 4)
             Address     0023.04ee.be01
             Hello Time  2  sec  Max Age 20 sec  Forward Delay 15 sec

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Po999            Root FWD 4         128.5094 (vPC peer-link) Network P2p 
Eth1/1           Desg FWD 4         128.1    P2p 
Eth1/2           Desg FWD 4         128.2    P2p 

BSN-SW2

BSN-SW2# show spanning-tree vlan 4

VLAN0004
  Spanning tree enabled protocol rstp
  Root ID    Priority    4100
             Address     0023.04ee.be01
             This bridge is the root
             Hello Time  2  sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    4100   (priority 4096 sys-id-ext 4)
             Address     0023.04ee.be01
             Hello Time  2  sec  Max Age 20 sec  Forward Delay 15 sec

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Po999            Desg FWD 4         128.5094 (vPC peer-link) Network P2p 
Eth1/1           Desg FWD 4         128.1    P2p 
Eth1/2           Desg FWD 4         128.2    P2p 

PHX-SW1

PHX-SW1# show spanning-tree vlan 4

VLAN0004
  Spanning tree enabled protocol rstp
  Root ID    Priority    4100
             Address     0023.04ee.be01
             Cost        4
             Port        1 (Ethernet1/1)
             Hello Time  2  sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32772  (priority 32768 sys-id-ext 4)
             Address     5254.0018.7292
             Hello Time  2  sec  Max Age 20 sec  Forward Delay 15 sec

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Eth1/1           Root FWD 4         128.1    P2p 
Eth1/2           Altn BLK 4         128.2    P2p 
Eth1/3           Altn BLK 4         128.3    P2p 
Eth1/4           Desg FWD 4         128.4    P2p 

PHX-SW2

PHX-SW2# show spanning-tree vlan 4

VLAN0004
  Spanning tree enabled protocol rstp
  Root ID    Priority    4100
             Address     0023.04ee.be01
             Cost        4
             Port        1 (Ethernet1/1)
             Hello Time  2  sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32772  (priority 32768 sys-id-ext 4)
             Address     5254.0008.b08b
             Hello Time  2  sec  Max Age 20 sec  Forward Delay 15 sec

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Eth1/1           Root FWD 4         128.1    P2p 
Eth1/2           Desg FWD 4         128.2    P2p 
Eth1/3           Desg FWD 4         128.3    P2p 

Answer 1

xSTP is not a good choice when the link status (up/down) does not reflect actual L2 connectivity.

Your diagram shows an L2 connection between BSN-1 and PHX-1 across different ISPs. Since that is very unlikely, you seem to have some L2 tunneling going on there (VPN?). That tunneling possibly hides the actual link status when connectivity is lost, at least for some time.

You should consider switching to routed L3 links that don't require xSTP for removing bridge loops. Assuming you're using VPN between locations, OSPF between the routers should provide more reliable link status and route propagation.