This is a known Elasticsearch version downgrade issue where you cannot downgrade once data has been written with a newer version due to Lucene codec incompatibility. I found the following relevant resources:
- Elasticsearch version downgrade troubleshooting
- Camunda upgrade preparation - verify infrastructure compatibility
- Elasticsearch 7 to 8 compatibility changes
- Camunda 8.8 Elasticsearch requirements
- Zeebe data loss troubleshooting - PVC deletion
- Camunda Helm chart uninstalling guide
Does this help? If not, can anyone from the community jump in? 
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