Incorrect bar ordering for sub-bucket term aggregation and custom metric as "Order by"

[mkurtak]

Kibana version: 6.3.2

Elasticsearch version: 6.3.2

Server OS version: Centos 7

Browser version: Chrome

Original install method (e.g. download page, yum, from source, etc.): yum

Describe the bug:
Incorrect bar ordering in vertical chart when using "Terms" aggregation ordered by "Custom metric" based on timefield and combined with Split series sub-aggregation using another Term.

Steps to reproduce:

1. Create vertical bar chart visualization
2. Put Count on Y-Axis
3. Put Terms aggregation on X-Axis
4. Set Order By to Custom Metric with Max Aggregation on Field of type date (each record in term bucket has same value of this field, but each bucket has different value)
5. Vertical bar chart has correct orders - first screenshot
6. Add Split Series as another Term Sub Aggregation (not every bucket on X-Axis contains all subbucket terms)
7. Vertical bar chart has incorrect orders. It seems that only buckets not containing all sub-bucket terms are incorrectly sorted

Additional info:
I've found issue #3314, but it seems fixed two years ago, so I'm not sure if its a same bug.

Screenshots

Correct bucket order:

Incorrect bucket order:

[timroes]

Duplicate of #17532