[Downgrade] Swap rebalance fails when downgrading cluster from 7.6.0 to 7.1.0

Steps:

1. Couchbase version 7.1.0-2556 is initially installed on 3 nodes and a cluster is initialised with these 3 nodes running just the KV service.
2. Create a Magma bucket with 1GB RAM per node and 1 replica.
3. 5 millions documents/items are loaded into the bucket.
4. Couchbase version 7.6.0-1980 is installed on the spare node and this spare node is swap rebalanced with a node in the cluster.
5. The above step is performed another time until 2 nodes in the cluster are in 7.6.0 and 1 node is in 7.1.0.
6. Now, 7.1.0-2556 is installed on the spare node and swap rebalance of this is performed with a 7.6.0 node in the cluster. This is the start of the downgrade process.
7. 172.23.217.175 (7.6.0, going out) is swap rebalanced with 172.23.217.179 (7.1.1, coming in)
   Swap rebalance fails with the reason

{mover_crashed,

{unexpected_exit,

{'EXIT',<0.29005.3>,

{{{{{child_interrupted,

{'EXIT',<34812.10738.0>,socket_closed}},

[{dcp_replicator,spawn_and_wait,1,

[{file,"src/dcp_replicator.erl"},

{line,358}]},

{dcp_replicator,handle_call,3,

[{file,"src/dcp_replicator.erl"},

{line,146}]},

{gen_server,try_handle_call,4,

[{file,"gen_server.erl"},{line,721}]},

{gen_server,handle_msg,6,

[{file,"gen_server.erl"},{line,750}]},

{proc_lib,init_p_do_apply,3,

[{file,"proc_lib.erl"},{line,226}]}]},

{gen_server,call,

[<34812.10735.0>,get_partitions,

infinity]}},

{gen_server,call,

['dcp_replication_manager-bucket-0',

{get_replicator_pid,236},

infinity]}},

{gen_server,call,

[{'janitor_agent-bucket-0',

'ns_1@172.23.217.179'},

{if_rebalance,<0.9249.3>,

{update_vbucket_state,237,active,

undefined,undefined,

[['ns_1@172.23.217.179',

'ns_1@172.23.217.180']]}},

infinity]}}}}}. 

1. Cb-collect logs have been attached.