When the following command is configured for the backup slot ports, it seems like the LACP PDUs are sent with the VLAN tag value of 0. So, the length of the PDU is 132 instead of 128 bytes and hence the third party devices ignore these PDUs.
enable vman cep egress filtering port <port number>
Ports from the same LACP sharing group is added to the aggregator if it is from the master node. Issue is seen only with the backup slot of a stack. This is identified as a bug. CR xos0069058 is created to fix this issue