Thanks @nateynate, I’m glad I figured it out now!
The problem was caused by an index template that I created manually, targeting the same index pattern (otel-v1-apm-span-*) that Data Prepper uses by default for traces. I wanted to customize certain index settings and mappings, but I didn’t realize that this would completely override the default template that Data Prepper automatically applies (as described in the Data Prepper documentation).
Since Data Prepper uses a default template type of v1 if none is explicitly set (template_type defaults to v1), my manual template ended up taking precedence. Unfortunately, my template didn’t include all the necessary mappings - especially important ones like mapping serviceName as a keyword field. This mismatch broke the Observability UI, causing the traces to be ingested but not displayed.
To fix it, I just deleted the custom template I’d created. After that, I reindexed the existing trace data into fresh indices, so they could pick up the correct mappings from Data Prepper’s default template. Everything started working again immediately, and now the traces show up correctly in the Observability UI
After these steps, everything started working normally again and the traces are now properly visible in the Observability UI.
I hope this helps someone else who might be experiencing the same problem!