Split series alphabetical order is not respected

[akarzazi]

Kibana version:
docker.elastic.co/kibana/kibana:6.6.1
Elasticsearch version:
docker.elastic.co/elasticsearch/elasticsearch:6.6.1
Server OS version:
Docker
Browser version:
Chrome 72
Browser OS version:
windows
Original install method (e.g. download page, yum, from source, etc.):

Describe the bug:

In the sub aggregation (split series) the items that do not appear in the first range are not sorted correctly
Below you can see that the "priority" "2", is not sorted.

Note that the issue appears only when the sort is ascending.

Steps to reproduce:

1. Create a main bucket as histogram
2. Create a split series with an alphabetical ascending sort
3. Make sure one of the item of the split series does not appear in the first range of the main histogram

[elasticmachine]

Pinging @elastic/kibana-app

[timroes]

@lukeelmers since you've already done some analysis, is this a duplicate of #17532?

[lukeelmers]

Yes, this sounds like the same issue. Basically the ordering of subbuckets is based on the results of the first top-level bucket.

@akarzazi Feel free to follow along in #17532 for any updates and a more detailed explanation. I’ll go ahead and close this one.