Loopback-detect function not supported over trunk interface

Customer have the topology below with STP disabled. The switches are S5720-52X-PWR-LI-AC model with version V200R010C00SPC600 On top of this topology with stp disabled, he configured loopback-detect function for all interfaces except interfaces GigabitEthernet 0/0/17 and 0/0/18 which are added in Eth-Trunk 1 interface for both E2 and E3. Switch E1 is a HP switch with loopback-detect function enabled also on his Bridge-Aggregation interfaces.

After connecting by mistake, switch E2 interface GigabitEthernet 0/0/4 with switch E3 interface GigabitEthernet 0/0/9, a loop was created at layer two.

E2 interface:

#

interface GigabitEthernet0/0/4

 port link-type access

 port default vlan 168

 loopback-detect packet vlan 168

 loopback-detect enable

#

E3 interface

#

interface GigabitEthernet0/0/9

 port link-type access

 port default vlan 168

 loopback-detect packet vlan 168

 loopback-detect enable

#

Customer reported this incident and asked why is this possible since the two interfaces were working in loopback-detect mechanism.

Apparently the loopback-detect function does not work over Eth-Trunk interfaces. We have tried also to enable loopback-detect on physical interfaces that are added to aggregation interface but, as soon as the interfaces are added to Eth-Trunk, the command does not work anymore.

The conclusion is that the function is not supported over Eth-Trunk interfaces.