I'm using the APM Python agent with a Django application. After migrating from Opbeat to Elastic APM I've been missing transactions from many Django views in the views table (in the APM UI in Kibana). They are, however, shown when explicitly filtered for and they are always represented in the graph showing requests per minute (unless filtered out). It did seem that all affected views were Django REST Framework views but that might not be true. Not all but the vast majority of the missing transactions have a HTTP 4xx result. The views with missing transactions should have very high impact since there are thousands of requests per minute for those ones, in contrast to many other views.
To me, this appears to be a bug in the APM UI as all transactions still are indexed and available when filtered or searched for.
Oh, I'm running Elasticsearch, Kibana and APM versions 6.4.2. The Python agent is 2.1.1 but I guess that shouldn't be an issue.