In lacp_select_tx_port_by_hash(), we assert that the selected port is DISTRIBUTING. However, the port state is protected by the LACP_LOCK(), which is not held around lacp_select_tx_port_by_hash(). So this assertion is racy, and can result in a spurious panic when links are flapping.
It is certainly possible to fix it by acquiring LACP_LOCK(), but this seems like an early development assert, and it seems best to just remove it, rather than add complexity inside an ifdef INVARIANTS..