@alex just to follow up, it turns out that it’s actually both the scala stream enricher and scala collector. It does appear that the enricher indeed does not support text/plain (https://github.com/snowplow/snowplow/blob/master/3-enrich/scala-common-enrich/src/test/scala/com.snowplowanalytics.snowplow.enrich.common/adapters/registry/SnowplowAdapterSpec.scala#L162)
Still leaves the question about why events are being generated with this header, but oh well, I’m sure we can figure that out 
Thanks for all the help.