I think this is the same problem noted here: https://discuss.elastic.co/t/ingest-node-pipeline-with-id-x-does-not-exist-solved/54436
That thread says "solved" but I see no resolution. Just an inability to reproduce the error. We're still getting the error in later versions.
Using 2 custom processors
Single node cluster(s)
Has occurred in at least half a dozen clusters
I don't think I could do a better job at noting the specifics than @PatrickKik so I'll quote him: