Http 400 query crashes elastic search cluster

on a 5.6 ES cluster that i don't administrate, i noticed, when a query gives an error HTTP 400 such as

{"error":{"root_cause":[{"type":"action_request_validation_exception","reason":"Validation Failed: 1: template is missing;"}]

that it turns to yellow status, and while it re-assigns shards, there is no way to bulk some documents until it comes to 98.5% of active shards.

this is just correlation (i may make a mistake in my judgement)...

is this possible that coordinator nodes can't manage a mal-formed or not valid query ?

I don't think so.

But anyway your version is too old so you should upgrade to 6.8 or better 7.10

Thanks for your reply.
Yes, we aim for upgrade to 7.10.