During configuration change (vlan was removed) on one switch from MC-LAG pair, it was experienced connectivity loss due to MC-AE interfaces down on both switches in this MC-LAG pair in the same time.

Both devices are having "mc-lag consistency-check" feature configured, and the following activity was reported:

<SW1> show log messages | match lacpd

Feb 6 15:42:53 lacpd[7917]: interface ae1 changes from active to standby

Feb 6 15:42:53 lacpd[7917]: interface ae12 changes from active to standby

Feb 6 15:42:53 lacpd[7917]: interface ae2 changes from active to standby

Feb 6 15:42:53 lacpd[7917]: interface ae3 changes from active to standby

Feb 6 15:42:53 lacpd[7917]: mcae_conn_proto_prcs_iccp_msg_mcae_state_update, No mcae member existing for mcae id 5, rg id 1

Feb 6 15:42:53 lacpd[7917]: mcae_conn_proto_prcs_iccp_msg_mcae_state_update, No mcae member existing for mcae id 6, rg id 1

Feb 6 15:50:09 lacpd[7917]: interface ae5 changes from standby to active

Feb 6 15:50:09 lacpd[7917]: interface ae6 changes from standby to active

Feb 6 15:53:09 lacpd[7917]: interface ae1 changes from standby to active

Feb 6 15:53:09 lacpd[7917]: interface ae12 changes from standby to active

Feb 6 15:53:09 lacpd[7917]: interface ae3 changes from standby to active

Feb 6 15:53:09 lacpd[7917]: interface ae2 changes from standby to active

<SW2> show log messages | match lacpd

Feb 6 15:42:53 lacpd[7916]: interface ae1 changes from active to standby

Feb 6 15:42:53 lacpd[7916]: interface ae12 changes from active to standby

Feb 6 15:42:53 lacpd[7916]: interface ae2 changes from active to standby

Feb 6 15:42:53 lacpd[7916]: interface ae3 changes from active to standby

Feb 6 15:42:53 lacpd[7916]: interface ae5 changes from active to standby

Feb 6 15:42:53 lacpd[7916]: interface ae6 changes from active to standby

Feb 6 15:53:09 lacpd[7916]: interface ae12 changes from standby to active

Feb 6 15:53:09 lacpd[7916]: interface ae6 changes from standby to active

Feb 6 15:53:09 lacpd[7916]: interface ae5 changes from standby to active

Feb 6 15:53:09 lacpd[7916]: interface ae3 changes from standby to active

Feb 6 15:53:09 lacpd[7916]: interface ae2 changes from standby to active

Feb 6 15:53:09 lacpd[7916]: interface ae1 changes from standby to active

And the following related to mc-lag consistency-check:

<SW1>> show log messages | match CFGCHKD

Feb 6 15:42:53 w2sw034prsds1 mclag_cfgchkd[7951]: MCLAG_CFGCHKD_MANDATORY_CONSISTENCY_CHECK_FAILED: Mismatch detected in vlan membership config, error_code 78

<SW2>> show log messages | match CFGCHKD

Feb 6 15:42:53 w2sw035prsds2 mclag_cfgchkd[7947]: MCLAG_CFGCHKD_MANDATORY_CONSISTENCY_CHECK_FAILED: Mismatch detected in vlan membership config, error_code 78

As configuration was changed on only one MCLAG peer, the issue observed was expected. We can confirm the behavior reported on the following details:

How MC-LAG Consistency Checks Work

The following events take place during configuration consistency check after you issue a commit on the local MC-LAG peer:

1. Commit an MC-LAG configuration on the local MC-LAG peer.
2. ICCP parses the MC-LAG configuration and then sends the configuration to the remote MC-LAG peer.
3. The remote MC-LAG peer receives the MC-LAG configuration from the local MC-LAG peer and compares it with its own MC-LAG configuration.
4. If the there is a severe inconsistency between the two MC-LAG configurations, the MC-LAG interface is brought down, and syslog messages are issued.
5. If there is a moderate inconsistency between the two configurations, syslog messages are issued.

The following events take place during configuration consistency check after you issue a commit on the remote MC-LAG peer:

• Commit an MC-LAG configuration on the remote MC-LAG peer.
• ICCP parses the MC-LAG configuration and then sends the configuration to the local MC-LAG peer.
• The local MC-LAG peer receives the configuration from the remote MC-LAG peer and compares it with its own configuration.
• If the there is a severe inconsistency between the two configurations, the MC-LAG interface is brought down, and syslog messages are issued.
• If there is a moderate inconsistency between the two configurations, syslog messages are issued.

Reference link: https://www.juniper.net/documentation/us/en/software/junos/mc-lag/topics/topic-map/mc-lag-feature-additional-concepts.html#id-understanding-multichassis-link-aggregation-group-configuration-consistency-check