Custom reduce functions don't work in partitioned DB

## Description

In a partitioned DB using custom reduce function doesn't work, all queries to that view fail with 500 error. Creating exact same view in non-partitioned db works. Built in reduce funtions work fine though.

## Steps to Reproduce

1. Set up CouchDB cluster
2. Create partitioned and non-partitioned DBs
3. Apply the example view on both
4. Try to query in both databases
5. Swap custom reduce for _count built-in
6. Now works on both DBs

## Expected Behaviour

Custom reduce functions should work fine on partitioned DB or should be documented non-working

## Your Environment

Docker Compose setup with three nodes using Docker Hub `couchdb:3` image
* CouchDB version used: 3.2.1
* Browser name and version: Postman 9.4.12
* Operating system and version: Docker container on MacOS 10.15.7

## Additional Context

Design doc:
````
{
"views": {
"one": {
"map": "mapfun = function (doc) {\n if (doc.id !== \"\") {\n emit(doc.id, 1);\n }\n};\n",
"reduce": "function(keys, values, rereduce) {\n if (rereduce) {\n return sum(values);\n } else {\n return values.length;\n }\n}"
}
},
"filters": {},
"updates": {},
"language": "javascript",
"options": {}
}
````

Error returned:
````
{
"error": "{{invalid_ejson,{p,<<\"8a1d99e3-db1d-4a8a-a61a-bac8dad0d3ba\">>,\n <<\"01d5a505-192f-44f7-bf99-2960797f1956\">>}},\n [{jiffy,encode,2,[{file,\"src/jiffy.erl\"},{line,99}]},\n {couch_os_process,writejson,2,[{file,\"src/couch_os_process.erl\"},{line,97}]},\n {couch_os_process,handle_call,3,\n [{file,\"src/couch_os_process.erl\"},{line,200}]},\n {gen_server,try_handle_call,4,[{file,\"gen_server.erl\"},{line,636}]},\n {gen_server,handle_msg,6,[{file,\"gen_server.erl\"},{line,665}]},\n {proc_lib,init_p_do_apply,3,[{file,\"proc_lib.erl\"},{line,247}]}]}",
"reason": "{gen_server,call,\n [<17837.15344.57>,\n {prompt,[<<\"reduce\">>,\n [<<\"function(keys, values, rereduce) {\\n if (rereduce) {\\n return sum(values);\\n } else {\\n return values.length;\\n }\\n}\">>],\n [[[{p,<<\"8a1d99e3-db1d-4a8a-a61a-bac8dad0d3ba\">>,\n <<\"01d5a505-192f-44f7-bf99-2960797f1956\">>},\n <<\"8a1d99e3-db1d-4a8a-a61a-bac8dad0d3ba:01d5a505-192f-44f7-bf99-2960797f1956\">>],\n 1],\n [[{p,<<\"8a1d99e3-db1d-4a8a-a61a-bac8dad0d3ba\">>,\n <<\"02d1378c-f30d-4148-b5fd-3b216f612463\">>},\n <<\"8a1d99e3-db1d-4a8a-a61a-bac8dad0d3ba:02d1378c-f30d-4148-b5fd-3b216f612463\">>],\n 1],\n [[{p,<<\"8a1d99e3-db1d-4a8a-a61a-bac8dad0d3ba\">>,\n <<\"037362ca-f266-49d2-ab19-4cc946613de6\">>},\n <<\"8a1d99e3-db1d-4a8a-a61a-bac8dad0d3ba:037362ca-f266-49d2-ab19-4cc946613de6\">>],\n 1],\n [[{p,<<\"8a1d99e3-db1d-4a8a-a61a-bac8dad0d3ba\">>,\n <<\"04a2bae3-cd74-42fd-9d5d-113c827d69de\">>},\n <<\"8a1d99e3-db1d-4a8a-a61a-bac8dad0d3ba:04a2bae3-cd74-42fd-9d5d-113c827d69de\">>],\n 1],\n [[{p,<<\"8a1d99e3-db1d-4a8a-a61a-bac8dad0d3ba\">>,\n <<\"061f26ec-36fb-41b5-a361-44bb759f3c5b\">>},\n <<\"8a1d99e3-db1d-4a8a-a61a-bac8dad0d3ba:061f26ec-36fb-41b5-a361-44bb759f3c5b\">>],\n 1],\n [[{p,<<\"8a1d99e3-db1d-4a8a-a61a-bac8dad0d3ba\">>,\n <<\"0850305e-0859-4fe2-b65b-d4f96aca0366\">>},\n <<\"8a1d99e3-db1d-4a8a-a61a-bac8dad0d3ba:0850305e-0859-4fe2-b65b-d4f96aca0366\">>],\n 1],\n [[{p,<<\"8a1d99e3-db1d-4a8a-a61a-bac8dad0d3ba\">>,\n <<\"0f4146b5-6014-4907-931d-0dc82b0d365c\">>},\n <<\"8a1d99e3-db1d-4a8a-a61a-bac8dad0d3ba:0f4146b5-6014-4907-931d-0dc82b0d365c\">>],\n 1]]]},\n infinity]}",
"ref": 157986948
}
````
Docker Compose setup:

[docker setup.zip](https://github.com/apache/couchdb/files/8224151/docker.setup.zip)

Reproduce the partitioned and non-partitioned cases using the provided Docker Compose setup and the example design document. Inspect src/jiffy.erl and couch_os_process.erl around the reported invalid_ejson and reduce errors; done means custom reduce queries work on partitioned databases or the limitation is documented.

Written by the indexing model from the issue text.