Replication job crashes

We are trying to migrated some DBs between 2 CouchDB servers and if for some of the dbs things run smoothly for some bigger ones (15k docs tops) the replication jobs stop and from the _scheduler/docs we can see only the following errors reported:
info: {error: "{worker_died,<0.1956.3>,{bad_return_value,{invalid_json,{1,invalid_json}}}}"}
error: "{worker_died,<0.1956.3>,{bad_return_value,{invalid_json,{1,invalid_json}}}}"

## Description
The DBs are structurally the same but I'd like to find out what is the exact error or the document causing it.
We have also checked the server logs and the error reported is similar to the above one

* CouchDB version used: 3.2.0
* Browser name and version: Chrome
* Operating system and version: Ubuntu, Docker, K8S, Azure AKS

Start by reproducing replication on a larger database and inspect the _scheduler/docs output alongside the server logs. Trace the reported invalid_json worker failure to determine which document or input causes the job to stop. Done means the exact error or offending document is exposed clearly instead of only reporting worker_died.

Written by the indexing model from the issue text.