Courier fetch and index refresh

Hi,
I have added new index filebeat-* from filebeat directly to ES.
This new index does not have any filed "eventLogTime" in its mapping. So it shows "courier fetch: 3 of 1180 shards failed".

When I refreshed above index form ES-HQ then it works fine.

Can anybody explain me what might happened internally that solved issue?

br,
Sunil