Table visualization issue with Split table in column mode - Extra column added.

[jmartin51]

Kibana version:
7.8.1 , 7.9.0 at least
Elasticsearch version:
7.8.1 , 7.9.0 at least
Server OS version:

Browser version:
Chrome 84.0.4147.105

Describe the bug:
Table visualisation: When Split table is used in columns mode, the "splitting column" is repeated inside each rows of all tables, see below:

It should not be the case.

Indeed It is not the case using the Split tablewith rows mode. In this situation that is the expected behaviour:

It was working as expected also in previous version of Kibana like the 5.x

Steps to reproduce:
See my first screenshot, here the detailed steps:

1. Create a table visualizations in kibana, for example using the out of the box [Flights] Flight Log saved search
2. In Metrics Section use medianaggregation, choose a field, AvgTicketPrice in this example
3. In Bucket section, use Split table and Columns mode Aggregation Term and Field DestCityName.

Expected behavior:
Same behavior than using Split Table Rows Mode

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)