Toggling a column in a document's table view can remove existing columns

[bczifra]

Kibana version: 7.12.0

Elasticsearch version: N/A

Server OS version: N/A

Browser version: Chrome Version 89.0.4389.114 (Official Build) (x86_64)

Browser OS version: N/A

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

Describe the bug:
In Discover, Clicking the "Toggle column in table" option in a document's table view can cause already selected columns to be removed and replaced with the toggled column.

Steps to reproduce:

1. Load Discover
2. Expand a document
3. Add one or more fields using the left menu bar's "Add field as column" option.
4. Click the "Toggle column in table" option for one of the fields in the expanded document.
5. Note that the column(s) selected in Step 3 are now replaced by the field selected in Step 4.

Expected behavior:
The selected column is added to the already selected columns.

Screenshots (if relevant):

[elasticmachine]

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

[kertal]

thx for reporting @bczifra, fix is on the way