[TSVB] Fields lists do not populate all the times

[stratoula]

Kibana version:
master, 7.11

Describe the bug:
There are some occasions that the fields list and the time fields options don't populate.

I have tried to unskip the TSVB functional tests and this test https://github.com/elastic/kibana/blob/master/test/functional/apps/visualize/_tsvb_chart.ts#L133 fails a lot because the options on the list are not loaded.

Steps to reproduce:

1. Create a new TSVB chart
2. Change the default index pattern to another one
3. The time fields list options don't appear

This doesn't happen always but it happens a lot. Moreover it is a blocker to unskip the TSVB chart functional tests.

Expected behavior:
Fields should always appear on the list.

[elasticmachine]

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