In SSR with LAG interfaces, packet drops are seen after a configuration change.

Intermittent Ping request time out for LAG interfaces after config change

Currently at the moment, after any changes to the LAG configuration, you must restart the 128T service. Note that this step requires planned Maintenance Window (causes brief downtime)

# systemctl restart 128T

Please refer the section https://www.juniper.net/documentation/us/en/software/session-smart-router/docs/config_lacp/#configuring-lag-and-lacp which confirms this.