Upon further investigation this is a known issue for this specific version of Elasticsearch we are on.
- reindex causes java.lang.ArrayIndexOutOfBoundsException after upgrade to 8.7.1 · Issue #96078 · elastic/elasticsearch · GitHub
- Elasticsearch version 8.7.1 | Elasticsearch Guide [8.10] | Elastic
We are planning to upgrade to version 8.9.1 next week so hopefully this rectifies this issue.